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<div class="h1">
<h1 style="text-align: center">Subversion Design</h1>
</div>

<p class="warningmark"><em>NOTE: This document is out of date.  The last
  substantial update was in October 2002 (r3377).  However, people often come
  here for the section on the <a href="#server.fs.struct.bubble-up">directory
    bubble-up method</a>, which is still accurate.</em></p>

<div class="h1">
<h2>Table of Contents</h2>
<ol id="toc">
  <li><a href="#goals">Goals &mdash; The goals of the Subversion project</a>
  <ol>
    <li><a href="#goals.rename-remove-resurrect">Rename/removal/resurrection support</a></li>
    <li><a href="#goals.textbinary">Text vs binary issues</a></li>
    <li><a href="#goals.i18n">I18N/Multilingual support</a></li>
    <li><a href="#goals.branching-and-tagging">Branching and tagging</a></li>
    <li><a href="#goals.misc">Miscellaneous new behaviors</a>
    <ol>
      <li><a href="#goals.misc.logmsgs">Log messages</a></li>
      <li><a href="#goals.misc.diffplugins">Client side diff plug-ins</a></li>
      <li><a href="#goals.misc.merging">Better merging</a></li>
      <li><a href="#goals.misc.conflicts">Conflicts resolution</a></li>
    </ol>
    </li> <!-- goals.misc -->
  </ol>
  </li> <!-- goals -->
  <li><a href="#model">Model &mdash; The versioning model used by Subversion</a>
  <ol>
    <li><a href="#model.wc-and-repos">Working Directories and Repositories</a></li>
    <li><a href="#model.txns-and-revnums">Transactions and Revision Numbers</a></li>
    <li><a href="#model.how-wc">How Working Directories Track the Repository</a></li>
    <li><a href="#model.lock-merge">Locking vs. Merging - Two Paradigms of Co-operative
     Developments</a></li>
    <li><a href="#model.props">Properties</a></li>
    <li><a href="#model.merging-and-ancestry">Merging and Ancestry</a></li>
  </ol>
  </li> <!-- model -->
  <li><a href="#archi">Architecture &mdash; How Subversion's components work together</a>
  <ol>
    <li><a href="#archi.client">Client Layer</a></li>
    <li><a href="#archi.network">Network Layer</a></li>
    <li><a href="#archi.fs">Filesystem Layer</a></li>
  </ol>
  </li> <!-- archi -->
  <li><a href="#deltas">Deltas &mdash; How to describe changes</a>
  <ol>
    <li><a href="#deltas.text">Text Deltas</a></li>
    <li><a href="#deltas.prop">Property Deltas</a></li>
    <li><a href="#deltas.tree">Tree Deltas</a></li>
    <li><a href="#deltas.postfix-text">Postfix Text Deltas</a></li>
    <li><a href="#deltas.serializing-via-editor">Serializing Deltas via the "Editor" Interface</a></li>
  </ol>
  </li> <!-- deltas -->
  <li><a href="#client">Client &mdash; How the client works</a>
  <ol>
    <li><a href="#client.wc">Working copies and the working copy library</a>
    <ol>
      <li><a href="#client.wc.layout">The layout of working copies</a></li>
      <li><a href="#client.wc.library">The working copy management library</a></li>
    </ol>
    </li> <!-- client.wc -->
    <li><a href="#client.libsvn_ra">The repository access library</a></li>
    <li><a href="#client.libsvn_client">The client operation library</a></li>
  </ol>
  </li> <!-- client -->
  <li><a href="#protocol">Protocol &mdash; How the client and server communicate</a>
  <ol>
    <li><a href="#protocol.webdav">The HTTP/WebDAV/DeltaV based protocol</a></li>
    <li><a href="#protocol.svn">The custom protocol</a></li>
  </ol>
  </li> <!-- protocol -->
  <li><a href="#server">Server &mdash; How the server works</a>
  <ol>
    <li><a href="#server.fs">Filesystem</a>
    <ol>
      <li><a href="#server.fs.overview">Filesystem Overview</a></li>
      <li><a href="#server.fs.api">API</a></li>
      <li><a href="#server.fs.struct">Repository Structure</a>
      <ol>
        <li><a href="#server.fs.struct.schema">Schema</a></li>
        <li><a href="#server.fs.struct.bubble-up">Bubble-Up Method</a></li>
        <li><a href="#server.fs.struct.diffy-storage">Diffy Storage</a></li>
      </ol>
      </li> <!-- server.fs.struct -->
      <li><a href="#server.fs.implementation">Implementation</a></li>
    </ol>
    </li> <!-- server.fs -->
    <li><a href="#server.libsvn_repos">Repository Library</a></li>
  </ol>
  </li> <!-- server -->
  <li><a href="#license">License &mdash; Copyright</a></li>
</ol>
</div>

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  <div class="h2" id="goals" title="#goals">
  <h2>Goals &mdash; The goals of the Subversion project</h2>
  

  
    <p>The goal of the Subversion project is to write a version control
      system that takes over CVS's current and future user base
      
    (If you're not familiar with CVS or its shortcomings, then
          skip to <a href="#model">Model &mdash; The versioning model used by Subversion</a>)
  . The first release
      has all the major features of CVS, plus certain new features that CVS
      users often wish they had.  In general, Subversion works like CVS, except
      where there's a compelling reason to be different.</p>

    <p>So what does Subversion have that CVS doesn't?</p>

    <ul>
      <li><p>It versions directories, file-metadata, renames, copies
          and removals/resurrections.  In other words, Subversion records the
          changes users make to directory trees, not just changes to file
          contents.</p></li>

      <li><p>Tagging and branching are constant-time and
          constant-space.</p></li>

      <li><p>It is natively client-server, hence much more
          maintainable than CVS. (In CVS, the client-server protocol was added
          as an afterthought. This means that most new features have to be
          implemented twice, or at least more than once: code for the local
          case, and code for the client-server case.)</p></li>

      <li><p>The repository is organized efficiently and
          comprehensibly.  (Without going into too much detail, let's just say
          that CVS's repository structure is showing its
          age.)</p></li>

      <li><p>Commits are atomic.  Each commit results in a single
          revision number, which refers to the state of the entire tree.  Files
          no longer have their own revision numbers.</p></li>

      <li><p>The locking scheme is only as strict as absolutely
          necessary. Reads are never locked, and writes lock only the files
          being written, for only as long as needed.</p></li>

      <li><p>It has internationalization support.</p></li>

      <li><p>It handles binary files gracefully (experience has shown
          that CVS's binary file handling is prone to user
          error).</p></li>

      <li><p>It takes advantage of the Net's experience with CVS by
          choosing better default behaviors for certain
          situations.</p></li>
    </ul>

    <p>Some of these advantages are clear and require no further discussion.
      Others are not so obvious, and are explained in greater detail
      below.</p>
  

  <div class="h3" id="goals.rename-remove-resurrect" title="#goals.rename-remove-resurrect">
    <h3>Rename/removal/resurrection support</h3>
    

    <p>Full rename support means you can trace through ancestry by name
      <em>or</em> by entity.  For example, if you say "Give me
      revision 12 of foo.c", do you mean revision 12 of the file whose name is
      <em>now</em> foo.c (but perhaps it was named bar.c back at
      revision 12), or the file whose name was foo.c in revision 12 (perhaps
      that file no longer exists, or has a different name now)?  In Subversion,
      both interpretations are available to the user.</p>

    <p>(Note:  we've not yet implemented this, but it wouldn't be too hard.
      People are advocating switches to 'svn log' that cause history to be
      traced backwards either by entity or by path.)</p>
  </div> <!-- goals.rename-remove-resurrect (h3) -->

  <div class="h3" id="goals.textbinary" title="#goals.textbinary">
    <h3>Text vs binary issues</h3>
    

    <p>Historically, binary files have been problematic in CVS for two
      unrelated reasons: keyword expansion, and line-end conversion.</p>

    <ul>
      <li><p><strong class="firstterm">Keyword expansion</strong> is when CVS
          expands "$Revision$" into "$Revision: 1.1 $", for example.  There
          are a number of keywords in CVS: "$Author: sussman $", "$Date:
          2001/06/04 22:00:52 $", and so on.</p></li>
      <li><p><strong class="firstterm">Line-end conversion</strong> is when CVS
          gives plaintext files the appropriate line-ending conventions for the
          working copy's platform. For example, Unix working copies use LF, but
          Windows working copies use CRLF.  (Like CVS, the Subversion
          repository stores text files in Unix LF format).</p></li>
    </ul>

    <p>Both keyword substitution and line-end conversion are sensible only
      for plain text files.  CVS only recognizes two file types anyway:
      plaintext and binary.  And CVS assumes files are plain text unless you
      tell it otherwise.</p>

    <p>Subversion recognizes the same two types.  The question is, how does
      it determine a file's type?  Experience with CVS suggests that assuming
      text unless told otherwise is a losing strategy &ndash; people frequently
      forget to mark images and other opaque formats as binary, then later they
      wonder why CVS mangled their data.  So Subversion will not mangle data:
      when moving over the network, or when being stored in the repository, it
      treats all files as binary.  In the working copy, a tweakable meta-data
      property indicates whether to treat the file as text or binary for
      purposes of whether or not to allow contextual merging during
      updates.</p>

    <p>Users can turn line-end conversion on or off per file by tweaking
      meta-data.  Files do <em>not</em> undergo keyword
      substitution by default, on the theory that if someone wants substitution
      and isn't getting it, they'll look in the manual; but if they are getting
      it and didn't want it, they might just be confused and not know what to
      do.  Users can turn substitution on or off per file.</p>

    <p>Both of these changes are done on the client side; the repository
      does not even know about them.</p>
  </div> <!-- goals.textbinary (h3) -->

  <div class="h3" id="goals.i18n" title="#goals.i18n">
    <h3>I18N/Multilingual support</h3>
    

    <p>Subversion is internationalized &ndash; commands, user messages, and
      errors can be customized to the appropriate human language at build-time
      (or run time, if that's not much harder).</p>

    <p>File names and contents may be multilingual; Subversion does not
      assume an ASCII-only universe.  For purposes of keyword expansion and
      line-end conversion, Subversion also understands the UTF-* encodings (but
      not necessarily all of them by the first release).</p>
  </div> <!-- goals.i18n (h3) -->

  <div class="h3" id="goals.branching-and-tagging" title="#goals.branching-and-tagging">
    <h3>Branching and tagging</h3>
    
  
    <p>Subversion supports branching and tagging with one efficient
      operation: `clone'.  To clone a tree is to copy it, to create another
      tree exactly like it (except that the new tree knows its ancestry
      relationship to the old one).</p>

    <p>At the moment of creation, a clone requires only a small, constant
      amount of space in the repository &ndash; most of its storage is shared
      with the original tree.  If you never commit anything on the clone, then
      it's just like a CVS tag.  If you start committing on it, then it's a
      branch.  Voila!  This also implies CVS's "vendor branching" feature,
      since Subversion has real rename and directory support.</p>
  </div> <!-- goals.branching-and-tagging (h3) -->

  <div class="h3" id="goals.misc" title="#goals.misc">
    <h3>Miscellaneous new behaviors</h3>
    

    <div class="h4" id="goals.misc.logmsgs" title="#goals.misc.logmsgs">
      <h4>Log messages</h4>
      

      <p>Subversion has a flexible log message policy (a small matter, but
        one dear to our hearts).</p>

      <p>Log messages should be a matter of project policy, not version
        control software policy.  If a user commits with no log message, then
        Subversion defaults to an empty message.  (CVS tries to require log
        messages, but fails: we've all seen empty log messages in CVS, where
        the user committed with deliberately empty quotes.  Let's stop the
        madness now.)</p>
    </div> <!-- goals.misc.logmsgs (h4) -->

    <div class="h4" id="goals.misc.diffplugins" title="#goals.misc.diffplugins">
      <h4>Client side diff plug-ins</h4>
      

      <p>Subversion supports client-side plug-in diff programs.</p>

      <p>There is no need for Subversion to have every possible diff
        mechanism built in.  It can invoke a user-specified client-side diff
        program on the two revisions of the file(s) locally.</p>

      <p>(Note:  This feature does not exist yet, but is planned for
        post-1.0.)</p>
    </div> <!-- goals.misc.diffplugins (h4) -->

    <div class="h4" id="goals.misc.merging" title="#goals.misc.merging">
      <h4>Better merging</h4>
      

      <p>Subversion remembers what has already been merged in and what
        hasn't, thereby avoiding the problem, familiar to CVS users, of
        spurious conflicts on repeated merges.</p>

      <p>(Note: Parts of his feature (<a href="/merge-tracking/">Merge
          Tracking</a>) are implemented in Subversion&nbsp;1.5; see
          the <a href="svn_1.5_releasenotes.html#merge-tracking"
          >release notes</a>.)</p>

      <p>For details, see <a href="#model.merging-and-ancestry">Merging and Ancestry</a>.</p>
    </div> <!-- goals.misc.merging (h4) -->

    <div class="h4" id="goals.misc.conflicts" title="#goals.misc.conflicts">
      <h4>Conflicts resolution</h4>
      

      <p>For text files, Subversion resolves conflicts similarly to CVS, by
        folding repository changes into the working files with conflict
        markers.  But, for <em>both</em> text and binary files,
        Subversion also always puts the old and new pristine repository
        revisions into temporary files, and the pristine working copy revision
        in another temporary file.</p>

      <p>Thus, for any conflict, the user has four files readily at
        hand:</p>

      <ol>
        <li><p>the original working copy file with local
            mods</p></li>
        <li><p>the older repository file</p></li>
        <li><p>the newest repository file</p></li>
        <li><p>the merged file, with conflict
            markers</p></li>
      </ol>

      <p>and in a binary file conflict, the user has all but the
        last.</p>

      <p>When the conflict has been resolved and the working copy is
        committed, Subversion automatically removes the temporary pristine
        files.</p>

      <p>A more general solution would allow plug-in merge resolution tools
        on the client side; but this is not scheduled for the first release).
        Note that users can use their own merge tools anyway, since all the
        original files are available.</p>
    </div> <!-- goals.misc.conflicts (h4) -->
  </div> <!-- goals.misc (h3) -->
</div> <!-- goals (h2) -->

  <div class="h2" id="model" title="#model">
  <h2>Model &mdash; The versioning model used by Subversion</h2>
  

  
    <p>This chapter explains the user's view of Subversion &mdash; what
      &ldquo;objects&rdquo; you interact with, how they behave, and how they
      relate to each other.</p>
  

  <div class="h3" id="model.wc-and-repos" title="#model.wc-and-repos">
    <h3>Working Directories and Repositories</h3>
    

    <p>Suppose you are using Subversion to manage a software project.  There
      are two things you will interact with: your working directory, and the
      repository.</p>

    <p>Your <strong class="firstterm">working directory</strong> is an ordinary
      directory tree, on your local system, containing your project's sources.
      You can edit these files and compile your program from them in the usual
      way.  Your working directory is your own private work area: Subversion
      never changes the files in your working directory, or publishes the
      changes you make there, until you explicitly tell it to do so.</p>

    <p>After you've made some changes to the files in your working
      directory, and verified that they work properly, Subversion provides
      commands to publish your changes to the other people working with you on
      your project.  If they publish their own changes, Subversion provides
      commands to incorporate those changes into your working directory.</p>

    <p>A working directory contains some extra files, created and maintained
      by Subversion, to help it carry out these commands.  In particular, these
      files help Subversion recognize which files contain unpublished changes,
      and which files are out-of-date with respect to others' work.</p>

    <p>While your working directory is for your use alone, the
      <strong class="firstterm">repository</strong> is the common public record you share
      with everyone else working on the project.  To publish your changes, you
      use Subversion to put them in the repository.  (What this means, exactly,
      we explain below.)  Once your changes are in the repository, others can
      tell Subversion to incorporate your changes into their working
      directories.  In a collaborative environment like this, each user will
      typically have their own working directory (or perhaps more than one),
      and all the working directories will be backed by a single repository,
      shared amongst all the users.</p>

    <p>A Subversion repository holds a single directory tree, and records
      the history of changes to that tree.  The repository retains enough
      information to recreate any prior state of the tree, compute the
      differences between any two prior trees, and report the relations between
      files in the tree &mdash; which files are derived from which other
      files.</p>

    <p>A Subversion repository can hold the source code for several
      projects; usually, each project is a subdirectory in the tree.  In this
      arrangement, a working directory will usually correspond to a particular
      subtree of the repository.</p>

    <p>For example, suppose you have a repository laid out like this:</p>

    <pre>
/trunk/paint/Makefile
             canvas.c
             brush.c
       write/Makefile
             document.c
             search.c
</pre>

    <p>In other words, the repository's root directory has a single
      subdirectory named <tt class="filename">trunk</tt>, which itself contains two
      subdirectories: <tt class="filename">paint</tt> and
      <tt class="filename">write</tt>.</p>

    <p>To get a working directory, you must <strong class="firstterm">check out</strong>
      some subtree of the repository.  If you check out
      <tt class="filename">/trunk/write</tt>, you will get a working directory like
      this:</p>

    <pre>
write/Makefile
      document.c
      search.c
      .svn/
</pre>

    <p>This working directory is a copy of the repository's
      <tt class="filename">/trunk/write</tt> directory, with one additional entry
      &mdash; <tt class="filename">.svn</tt> &mdash; which holds the extra
      information needed by Subversion, as mentioned above.</p>

    <p>Suppose you make changes to <tt class="filename">search.c</tt>.  Since the
      <tt class="filename">.svn</tt> directory remembers the file's modification
      date and original contents, Subversion can tell that you've changed the
      file.  However, Subversion does not make your changes public until you
      explicitly tell it to.</p>

    <p>To publish your changes, you can use Subversion's
      &lsquo;<tt class="literal">commit</tt>&rsquo; command:</p>

    <pre>
$ pwd
/home/jimb/write
$ ls -a
.svn/   Makefile   document.c    search.c
$ svn commit search.c
$
</pre>

    <p>Now your changes to <tt class="filename">search.c</tt> have been committed
      to the repository; if another user checks out a working copy of
      <tt class="filename">/trunk/write</tt>, they will see your text.</p>

    <p>Suppose you have a collaborator, Felix, who checked out a working
      directory of <tt class="filename">/trunk/write</tt> at the same time you did.
      When you commit your change to <tt class="filename">search.c</tt>, Felix's
      working copy is left unchanged; Subversion only modifies working
      directories at the user's request.</p>

    <p>To bring his working directory up to date, Felix can use the
      Subversion &lsquo;<tt class="literal">update</tt>&rsquo; command.  This will
      incorporate your changes into his working directory, as well as any
      others that have been committed since he checked it out.</p>

    <pre>
$ pwd
/home/felix/write
$ ls -a
.svn/    Makefile    document.c    search.c
$ svn update
U search.c
$
</pre>

    <p>The output from the &lsquo;<tt class="literal">svn update</tt>&rsquo;
      command indicates that Subversion updated the contents of
      <tt class="filename">search.c</tt>.  Note that Felix didn't need to specify
      which files to update; Subversion uses the information in the
      <tt class="filename">.svn</tt> directory, and further information in the
      repository, to decide which files need to be brought up to date.</p>

    <p>We explain below what happens when both you and Felix make changes to
      the same file.</p>
  </div> <!-- model.wc-and-repos (h3) -->

  <div class="h3" id="model.txns-and-revnums" title="#model.txns-and-revnums">
    <h3>Transactions and Revision Numbers</h3>
    

    <p>A Subversion &lsquo;<tt class="literal">commit</tt>&rsquo; operation can
      publish changes to any number of files and directories as a single atomic
      transaction.  In your working directory, you can change files' contents,
      create, delete, rename and copy files and directories, and then commit
      the completed set of changes as a unit.</p>

    <p>In the repository, each commit is treated as an atomic transaction:
      either all the commit's changes take place, or none of them take place.
      Subversion tries to retain this atomicity in the face of program crashes,
      system crashes, network problems, and other users' actions.  We may call
      a commit a <strong class="firstterm">transaction</strong> when we want to emphasize
      its indivisible nature.</p>

    <p>Each time the repository accepts a transaction, this creates a new
      state of the tree, called a <strong class="firstterm">revision</strong>.  Each
      revision is assigned a unique natural number, one greater than the number
      of the previous revision.  The initial revision of a freshly created
      repository is numbered zero, and consists of an empty root
      directory.</p>

    <p>Since each transaction creates a new revision, with its own number,
      we can also use these numbers to refer to transactions; transaction
      <em class="replaceable">n</em> is the transaction which created revision
      <em class="replaceable">n</em>.  There is no transaction numbered
      zero.</p>

    <p>Unlike those of many other systems, Subversion's revision numbers
      apply to an entire tree, not individual files.  Each revision number
      selects an entire tree.</p>

    <p>It's important to note that working directories do not always
      correspond to any single revision in the repository; they may contain
      files from several different revisions.  For example, suppose you check
      out a working directory from a repository whose most recent revision is
      4:</p>

    <pre>
write/Makefile:4
      document.c:4
      search.c:4
</pre>

    <p>At the moment, this working directory corresponds exactly to revision
      4 in the repository.  However, suppose you make a change to
      <tt class="filename">search.c</tt>, and commit that change.  Assuming no other
      commits have taken place, your commit will create revision 5 of the
      repository, and your working directory will look like this:</p>

    <pre>
write/Makefile:4
      document.c:4
      search.c:5
</pre>

    <p>Suppose that, at this point, Felix commits a change to
      <tt class="filename">document.c</tt>, creating revision 6.  If you use
      &lsquo;<tt class="literal">svn update</tt>&rsquo; to bring your working
      directory up to date, then it will look like this:</p>

    <pre>
write/Makefile:6
      document.c:6
      search.c:6
</pre>

    <p>Felix's changes to <tt class="filename">document.c</tt> will appear in
      your working copy of that file, and your change will still be present in
      <tt class="filename">search.c</tt>.  In this example, the text of
      <tt class="filename">Makefile</tt> is identical in revisions 4, 5, and 6, but
      Subversion will mark your working copy with revision 6 to indicate that
      it is still current.  So, after you do a clean update at the root of your
      working directory, your working directory will generally correspond
      exactly to some revision in the repository.</p>
  </div> <!-- model.txns-and-revnums (h3) -->

  <div class="h3" id="model.how-wc" title="#model.how-wc">
    <h3>How Working Directories Track the Repository</h3>
    

    <p>For each file in a working directory, Subversion records two
      essential pieces of information:</p>

    <ul>
      <li><p>what revision of what repository file your working copy
          is based on (this is called the file's <strong class="firstterm">base
            revision</strong>), and</p></li>
      <li><p>a timestamp recording when the local copy was last
          updated.</p></li>
    </ul>

    <p>Given this information, by talking to the repository, Subversion can
      tell which of the following four states a file is in:</p>

    <ul>
      <li><p><strong>Unchanged, and current.</strong>
          The file is unchanged in the working directory, and no changes to that
          file have been committed to the repository since its base
          revision.</p></li>
      <li><p><strong>Locally changed, and
            current</strong>.  The file has been changed in the working
          directory, and no changes to that file have been committed to the
	  repository since its base revision.  There are local changes that have
	  not been committed to the repository.</p></li>
      <li><p><strong>Unchanged, and
            out-of-date</strong>.  The file has not been changed in
          the working directory, but it has been changed in the repository.  The
	  file should eventually be updated, to make it current with the
	  public revision.</p></li>
      <li><p><strong>Locally changed, and
            out-of-date</strong>.  The file has been changed both in the
          working directory, and in the repository.  The file should be updated;
	  Subversion will attempt to merge the public changes with the local
	  changes.  If it can't complete the merge in a plausible
	  way automatically, Subversion leaves it to the user to resolve the
	  conflict.</p></li>
    </ul>
  </div> <!-- model.how-wc (h3) -->

  <div class="h3" id="model.lock-merge" title="#model.lock-merge">
    <h3>Locking vs. Merging - Two Paradigms of Co-operative
     Developments</h3>
    

    <p>By default, Subversion prefers the &ldquo;merging&rdquo; method of
      handling simultaneous editing by multiple users.  This means that
      Subversion does not prevent two users from making changes to the same
      file at the same time.  For example, if both you and Felix have checked
      out working directories of <tt class="filename">/trunk/write</tt>, Subversion
      will allow both of you to change <tt class="filename">write/search.c</tt> in
      your working directories.  Then, the following sequence of events will
      occur:</p>

    <ul>
      <li><p>Suppose Felix tries to commit his changes to
          <tt class="filename">search.c</tt> first.  His commit will succeed, and
          his text will appear in the latest revision in the
          repository.</p></li>
      <li><p>When you attempt to commit your changes to
          <tt class="filename">search.c</tt>, Subversion will reject your commit,
          and tell you that you must update <tt class="filename">search.c</tt> before
          you can commit it.</p></li>
      <li><p>When you update <tt class="filename">search.c</tt>, Subversion
          will try to merge Felix's changes from the repository with your local
          changes.  By default, Subversion merges as if it were applying a
          patch: if your local changes do not overlap textually with Felix's,
          then all is well; otherwise, Subversion leaves it to you to resolve
          the overlapping changes.  In either case, Subversion carefully
          preserves a copy of the original pre-merge text.</p></li>
      <li><p>Once you have verified that Felix's changes and your
          changes have been merged correctly, you can commit the new revision
          of <tt class="filename">search.c</tt>, which now contains everyone's
          changes.</p></li>
    </ul>

    <p>Some version control systems provide &ldquo;locks&rdquo;, which
      prevent others from changing a file once one person has begun working on
      it.  In our experience, merging is preferable to locks, because:</p>

    <ul>
      <li><p>changes usually do not conflict, so Subversion's behavior
          does the right thing by default, while locking can interfere with
          legitimate work;</p></li>
      <li><p>locking can prevent conflicts within a file, but not
          conflicts between files (say, between a C header file and another
          file that includes it), so it doesn't really solve the problem; and
          finally,</p></li>
      <li><p>people often forget that they are holding locks,
          resulting in unnecessary delays and friction.</p></li>
    </ul>

    <p>Of course, some kinds of files with rigid formats, like images or
      executables, are simply not mergeable.  To support this, Subversion
      allows users to customize its merging behavior on a per-file basis.
      Firstly, you can direct Subversion to refuse to merge changes to certain
      files, and simply present you with the two original texts to choose from.
      Secondly, in Subversion 1.2 and later, support for the
      &ldquo;locking&rdquo; method of working is also available, and individual
      files can be designated as requiring locking.</p>

    <p>(In the future, you may be able to direct Subversion to merge using a
      tool which respects the semantics of specific complex file
      formats.)</p>
  </div> <!-- model.lock-merge (h3) -->

  <div class="h3" id="model.props" title="#model.props">
    <h3>Properties</h3>
    

    <p>Files generally have interesting attributes beyond their contents:
      mime-types, executable permissions, EOL styles, and so on.  Subversion
      attempts to preserve these attributes, or at least record them, when
      doing so would be meaningful.  However, different operating systems
      support very different sets of file attributes: Windows NT supports
      access control lists, while Linux provides only the simpler traditional
      Unix permission bits.</p>

    <p>In order to interoperate well with clients on many different
      operating systems, Subversion supports <strong class="firstterm">property
        lists</strong>, a simple, general-purpose mechanism which clients
      can use to store arbitrary out-of-band information about files.</p>

    <p>A property list is a set of name / value pairs.  A property name is
      an arbitrary text string, expressed as a Unicode UTF-8 string,
      canonically decomposed and ordered.  A property value is an arbitrary
      string of bytes.  Property values may be of any size, but Subversion may
      not handle very large property values efficiently.  No two properties in
      a given a property list may have the same name.  Although the word `list'
      usually denotes an ordered sequence, there is no fixed order to the
      properties in a property list; the term `property list' is
      historical.</p>

    <p>Each revision number, file, directory, and directory entry in the
      Subversion repository, has its own property list.  Subversion puts these
      property lists to several uses:</p>

    <ul>
      <li><p>Clients can use properties to store file attributes, as
          described above.</p></li>
      <li><p>The Subversion server uses properties to hold attributes
          of its own, and allow clients to read and modify them.  For example,
          someday a hypothetical &lsquo;<tt class="literal">svn-acl</tt>&rsquo;
          property might hold an access control list which the Subversion server
          uses to regulate access to repository files.</p></li>
      <li><p>Users can invent properties of their own, to store
          arbitrary information for use by scripts, build environments, and so
          on.  Names of user properties should be URI's, to avoid conflicts
          between organizations.</p></li>
    </ul>

    <p>Property lists are versioned, just like file contents.  You can
      change properties in your working directory, but those changes are not
      visible in the repository until you commit your local changes.  If you do
      commit a change to a property value, other users will see your change
      when they update their working directories.</p>
  </div> <!-- model.props (h3) -->

  <div class="h3" id="model.merging-and-ancestry" title="#model.merging-and-ancestry">
    <h3>Merging and Ancestry</h3>
    

    <p>[WARNING:  this section was written in May 2000, at the very
      beginning of the Subversion project.  This functionality probably will
      not exist in Subversion 1.0, but it's planned for post-1.0.  The problem
      should be reasonably solvable by recording merge data in
      'properties'.]</p>

    <p>Subversion defines merges the same way CVS does: to merge means to
      take a set of previously committed changes and apply them, as a patch, to
      a working copy.  This change can then be committed, like any other
      change.  (In Subversion's case, the patch may include changes to
      directory trees, not just file contents.)</p>

    <p>As defined thus far, merging is equivalent to hand-editing the
      working copy into the same state as would result from the patch
      application.  In fact, in CVS there <em>is</em> no difference
      &ndash; it is equivalent to just editing the files, and there is no
      record of which ancestors these particular changes came from.
      Unfortunately, this leads to conflicts when users unintentionally merge
      the same changes again.  (Experienced CVS users avoid this problem by
      using branch- and merge-point tags, but that involves a lot of unwieldy
      bookkeeping.)</p>

    <p>In Subversion, merges are remembered by recording <strong class="firstterm">ancestry
        sets</strong>.  A revision's ancestry set is the set of all changes
      "accounted for" in that revision.  By maintaining ancestry sets, and
      consulting them when doing merges, Subversion can detect when it would
      apply the same patch twice, and spare users much bookkeeping.  Ancestry
      sets are stored as properties.</p>

    <p>In the examples below, bear in mind that revision numbers usually
      refer to changes, rather than the full contents of that revision.  For
      example, "the change A:4" means "the delta that resulted in A:4", not
      "the full contents of A:4".</p>

    <p>The simplest ancestor sets are associated with linear histories.  For
      example, here's the history of a file A:</p>

    <pre>

 _____        _____        _____        _____        _____
|     |      |     |      |     |      |     |      |     |
| A:1 |-----&gt;| A:2 |-----&gt;| A:3 |-----&gt;| A:4 |-----&gt;| A:5 |
|_____|      |_____|      |_____|      |_____|      |_____|

</pre>

    <p>The ancestor set of A:5 is:</p>

    <pre>

  { A:1, A:2, A:3, A:4, A:5 }

</pre>

    <p>That is, it includes the change that brought A from nothing to A:1,
      the change from A:1 to A:2, and so on to A:5.  From now on, ranges like
      this will be represented with a more compact notation:</p>

    <pre>

  { A:1-5 }

</pre>

    <p>Now assume there's a branch B based, or "rooted", at A:2.  (This
      postulates an entirely different revision history, of course, and the
      global revision numbers in the diagrams will change to reflect it.)
      Here's what the project looks like with the branch:</p>

    <pre>

 _____        _____        _____        _____        _____        _____
|     |      |     |      |     |      |     |      |     |      |     |
| A:1 |-----&gt;| A:2 |-----&gt;| A:4 |-----&gt;| A:6 |-----&gt;| A:8 |-----&gt;| A:9 |
|_____|      |_____|      |_____|      |_____|      |_____|      |_____|
                \
                 \
                  \  _____        _____        _____
                   \|     |      |     |      |     |
                    | B:3 |-----&gt;| B:5 |-----&gt;| B:7 |
                    |_____|      |_____|      |_____|

</pre>

    <p>If we produce A:9 by merging the B branch back into the
      trunk</p>

    <pre>

 _____        _____        _____        _____        _____        _____
|     |      |     |      |     |      |     |      |     |      |     |
| A:1 |-----&gt;| A:2 |-----&gt;| A:4 |-----&gt;| A:6 |-----&gt;| A:8 |---.-&gt;| A:9 |
|_____|      |_____|      |_____|      |_____|      |_____|  /   |_____|
                \                                            |
                 \                                           |
                  \  _____        _____        _____        /
                   \|     |      |     |      |     |      /
                    | B:3 |-----&gt;| B:5 |-----&gt;| B:7 |---&gt;-'
                    |_____|      |_____|      |_____|

</pre>

    <p>then what will A:9's ancestor set be?</p>

    <pre>

  { A:1, A:2, A:4, A:6, A:8, A:9, B:3, B:5, B:7}

</pre>

    <p>or more compactly:</p>

    <pre>

  { A:1-9, B:3-7 }

</pre>

    <p>(It's all right that each file's ranges seem to include non-changes;
      this is just a notational convenience, and you can think of the
      non-changes as either not being included, or being included but being
      null deltas as far as that file is concerned).</p>

    <p>All changes along the B line are accounted for (changes B:3-7), and
      so are all changes along the A line, including both the merge and any
      non-merge-related edits made before the commit.</p>

    <p>Although this merge happened to include all the branch changes, that
      needn't be the case.  For example, the next time we merge the B
      line</p>

    <pre>

 _____     _____     _____     _____     _____      _____      _____
|     |   |     |   |     |   |     |   |     |    |     |    |     |
| A:1 |--&gt;| A:2 |--&gt;| A:4 |--&gt;| A:6 |--&gt;| A:8 |-.-&gt;| A:9 |-.-&gt;|A:11 |
|_____|   |_____|   |_____|   |_____|   |_____| |  |_____| |  |_____|
             \                                  /          |
              \                                /           |
               \  _____     _____     _____   / _____      |
                \|     |   |     |   |     | / |     |    /
                 | B:3 |--&gt;| B:5 |--&gt;| B:7 |--&gt;|B:10 |-&gt;-'
                 |_____|   |_____|   |_____|   |_____|

</pre>

    <p>Subversion will know that A's ancestry set already contains B:3-7, so
      only the difference between B:7 and B:10 will be applied.  A's new
      ancestry will be</p>

    <pre>

  { A:1-11, B:3-10 }

</pre>

    <p>But why limit ourselves to contiguous ranges?  An ancestry set is
      truly a set &ndash; it can be any subset of the changes available:</p>

    <pre>

 _____        _____        _____        _____        _____        _____
|     |      |     |      |     |      |     |      |     |      |     |
| A:1 |-----&gt;| A:2 |-----&gt;| A:4 |-----&gt;| A:6 |-----&gt;| A:8 |--.--&gt;|A:10 |
|_____|      |_____|      |_____|      |_____|      |_____| /    |_____|
                |                                          /
                |                ______________________.__/
                |               /                      |
                |              /                       |
                \           __/_                      _|__
                 \         {    }                    {    }
                  \  _____        _____        _____        _____
                   \|     |      |     |      |     |      |     |
                    | B:3 |-----&gt;| B:5 |-----&gt;| B:7 |-----&gt;| B:9 |-----&gt;
                    |_____|      |_____|      |_____|      |_____|

</pre>

    <p>In this diagram, the change from B:3-5 and the change from B:7-9 are
      merged into a working copy whose ancestry set (so far) is
      {&nbsp;A:1-8&nbsp;} plus any local changes.  After committing, A:10's
      ancestry set is</p>

    <pre>

  { A:1-10, B:5, B:9 }

</pre>

    <p>Clearly, saying "Let's merge branch B into A" is a little ambiguous.
      It usually means "Merge all the changes accounted for in B's tip into A",
      but it <em>might</em> mean "Merge the single change that
      resulted in B's tip into A".</p>

    <p>Any merge, when viewed in detail, is an application of a particular
      set of changes &ndash; not necessarily adjacent ones &ndash; to a working
      copy.  The user-level interface may allow some of these changes to be
      specified implicitly.  For example, many merges involve a single,
      contiguous range of changes, with one or both ends of the range easily
      deducible from context (i.e., branch root to branch tip).  These
      inference rules are not specified here, but it should be clear in most
      contexts how they work.</p>

    <p>Because each node knows its ancestors, Subversion never merges the
      same change twice (unless you force it to).  For example, if after the
      above merge, you tell Subversion to merge all B changes into A,
      Subversion will notice that two of them have already been merged, and so
      merge only the other two changes, resulting in a final ancestry set
      of:</p>

    <pre>

  { A:1-10, B:3-9 }

</pre>

<!--
  Heh, what about this: 

    B:3 adds line 3, with the text "foo". 
    B:5 deletes line 3. 
    B:7 adds line 3, with the text "foo". 
    B:9 deletes line 3. 

  The user first merges B:5 and B:9 into A.  If A had that line, it goes away
  now, nothing more. 

  Next, user merges B:3 and B:7 into A.  The second merge must conflict.

  I'm not sure we need to care about this, I just thought I'd note how even
  merges that seem like they ought to be easily composable can still suck. :-)
-->

    <p>This description of merging and ancestry applies to both intra- and
      inter-repository merges.  However, inter-repository merging will probably
      not be implemented until a future release of Subversion.</p>
  </div> <!-- model.merging-and-ancestry (h3) -->
</div> <!-- model (h2) -->

  <div class="h2" id="archi" title="#archi">
  <h2>Architecture &mdash; How Subversion's components work together</h2>
  

  
    <p>Subversion is conceptually divided into a number of separable
      layers.</p>

    <p>Assuming that the programmatic interface of each layer is
      well-defined, it is easy to customize the different parts of the system.
      Contributors can write new client apps, new network protocols, new server
      processes, new server features, and new storage back-ends.</p>

    <p>The following diagram illustrates the "layered" architecture, and
      where each particular interface lies.</p>

    <pre>
                    +--------------------+
                    | commandline or GUI |
                    |    client app      |
         +----------+--------------------+----------+ &lt;=== Client interface
         |              Client Library              |
         |                                          |
         |        +----+                            |
         |        |    |                            |
 +-------+--------+    +--------------+--+----------+ &lt;=== Network interface
 | Working Copy   |    |    Remote    |  | Local    |
 | Management lib |    | Repos Access |  | Repos    |
 +----------------+    +--------------+  | Access   |
                       |     neon     |  |          |
                       +--------------+  |          |
                          ^              |          |
                         /               |          |
                   DAV  /                |          |
                       /                 |          |
                      v                  |          |
              +---------+                |          |
              |         |                |          |
              | Apache  |                |          |
              |         |                |          |
              +---------+                |          |
              | mod_DAV |                |          |
            +-------------+              |          |
            | mod_DAV_SVN |              |          |
 +----------+-------------+--------------+----------+ &lt;=== Filesystem interface
 |                                                  |
 |               Subversion Filesystem              |
 |                                                  |
 +--------------------------------------------------+

</pre>
  

  <div class="h3" id="archi.client" title="#archi.client">
    <h3>Client Layer</h3>
    

    <p>The Subversion client, which may be either
      command-line or GUI, draws on three libraries.</p>

    <p>The working copy library, <tt class="filename">libsvn_wc</tt>, provides
      an API for managing the client's working copy of a project.  This
      includes operations like renaming or removal of files, patching files,
      extracting local diffs, and routines for maintaining administrative
      files in the <tt class="filename">.svn/</tt> directory.</p>

    <p>The repository_access library, <tt class="filename">libsvn_ra</tt>,
      provides an API for exchanging information with a Subversion
      repository.  This includes the ability to read files, write new
      revisions of files, and ask the repository to compare a working copy
      against its latest revision.  Note that there are two implementations
      of this interface: one designed to talk to a repository over a network,
      and one designed to work with a repository on local disk.  Any number
      of interface implementations can exist.</p>

    <p>The client library, <tt class="filename">libsvn_client</tt> provides
      general client functions such as <tt class="literal">update()</tt> and
      <tt class="literal">commit()</tt>, which may involve one or both of the other
      two client libraries.  <tt class="filename">libsvn_client</tt> should, in
      theory, provide an API that allows anyone to write a Subversion client
      application.</p>

    <p>For details, see <a href="#client">Client &mdash; How the client works</a>.</p>
  </div> <!-- archi.client (h3) -->

  <div class="h3" id="archi.network" title="#archi.network">
    <h3>Network Layer</h3>
    

    <p> The network layer's job is to move the repository API requests
      over a wire.</p>

    <p>On the client side, a network library
      (<tt class="filename">libneon</tt>) translates these requests into a set of
      HTTP WebDAV/DeltaV requests.  The information is sent over TCP/IP to an
      Apache server.  Apache is used for the following reasons:</p>

    <ul>
      <li><p>it is time-tested and extremely
          stable;</p></li>
      <li><p>it has built-in load-balancing;</p></li>
      <li><p>it has built-in proxy and firewall
          support;</p></li>
      <li><p>it has authentication and encryption
          features;</p></li>
      <li><p>it allows client-side caching;</p></li>
      <li><p>it has an extensible module system</p></li>
    </ul>

    <p>Our rationale is that any attempt to write a dedicated "Subversion
      server" (with a "Subversion protocol") would inevitably end up evolving
      towards Apache's already-existing feature set.  (However, Subversion's
      layered architecture certainly doesn't <em>prevent</em>
      anyone from writing a totally new network access
      implementation.)</p>

    <p>An Apache module (<tt class="filename">mod_dav_svn</tt>) translates the
      DAV requests into API calls against a particular repository.</p>

    <p>For details, see <a href="#protocol">Protocol &mdash; How the client and server communicate</a>.</p>
  </div> <!-- archi.network (h3) -->

  <div class="h3" id="archi.fs" title="#archi.fs">
    <h3>Filesystem Layer</h3>
    

    <p>When the requests reach a particular repository, they are
      interpreted by the <strong class="firstterm">Subversion Filesystem
        library</strong>, <tt class="filename">libsvn_fs</tt>.  The Subversion
      Filesystem is a custom Unix-like filesystem, with a twist: writes are
      revisioned and atomic, and no data is ever deleted!  This filesystem is
      currently implemented on top of a normal filesystem, using Berkeley DB
      files.</p>

    <p>For a more detailed explanation: see <a href="#server">Server &mdash; How the server works</a>.</p>
  </div> <!-- archi.fs (h3) -->
</div> <!-- archi (h2) -->

  <div class="h2" id="deltas" title="#deltas">
  <h2>Deltas &mdash; How to describe changes</h2>
  

  
    <p>Subversion uses three kinds of deltas:</p>

    <ul>

      <li><p>A <strong><strong class="firstterm">tree
              delta</strong></strong> describes the difference between two
          arbitrary directory trees, the way a traditional patch describes the
          difference between two files.  For example, the delta between
          directories A and B could be applied to A, to produce B.</p>
        
        <p>Tree deltas can also carry ancestry information, indicating how
          the files in one tree are related to files in the other tree.  And
          deltas can describe changes to file meta-information, like permission
          bits, creation dates, and so on.  The repository and working copy use
          deltas to communicate changes.</p></li>

      <li><p>A <strong><strong class="firstterm">text
              delta</strong></strong> describes changes to a string of
          bytes, such as the contents of a file.  It is analogous to
          traditional patch format, except that it works equally well on binary
          and text files, and is not invertible (because context and deleted
          data are not recorded).</p></li>

      <li><p>A <strong><strong class="firstterm">property
              delta</strong></strong> describes changes to a list of named
          properties (see <a href="#model.props">Properties</a>).</p></li>
    </ul>

    <p>The term <strong class="firstterm">delta</strong> without qualification generally
      means a tree delta, unless some other meaning is clear from
      context.</p>

    <p>In the examples below, deltas will be described in XML, which happens
      to be Subversion's (now mostly defunct) import/export patch format.
      However, note that deltas are an abstract data structure, of which the
      XML format is merely one representation.  Later, we will describe other
      representations: for example, there is a serialized representation
      (useful for streaming protocols, among other things), and a db-style
      representation, used for repository storage.  The various representations
      of a given delta are (in theory, anyway) perfectly isomorphic to one
      another, since they describe the same underlying structure.</p>
  

  <div class="h3" id="deltas.text" title="#deltas.text">
    <h3>Text Deltas</h3>
    

    <p>A text delta describes the difference between two strings of bytes,
      the <strong class="firstterm">source</strong> string and the
      <strong class="firstterm">target</strong> string.  Given a source string and a target
      string, we can compute a text delta; given a source string and a delta,
      we can reconstruct the target string.  However, note that deltas are not
      invertible: you cannot always reconstruct the source string given the
      target string and delta.</p>

    <p>The standard Unix &ldquo;diff&rdquo; format is one possible
      representation for text deltas; however, diffs are not ideal for internal
      use by a revision control system, for several reasons:</p>

    <ul>
      <li><p>Diffs are line-oriented, which makes them human-readable,
          but sometimes makes them perform poorly on binary
          files.</p></li>
      <li><p>Diffs represent a series of replacements, exchanging
          selected ranges ofthe old text with new text; again, this is easy for
          humans to read, butit is more expensive to compute and less compact
          than some alternatives.</p></li>
    </ul>

    <p>Instead, Subversion uses the VDelta binary-diffing algorithm, as
      described in <em class="citetitle">Hunt, J. J., Vo, K.-P., and Tichy, W. F.  An
        empirical study of delta algorithms.  Lecture Notes in Computer Science
        1167 (July 1996), 49-66.</em>  Currently, the output of this
      algorithm is stored in a custom data format called
      <strong class="firstterm">svndiff</strong>, invented by Greg Hudson &lt;&gt;, a
      Subversion developer.</p>

    <p>The concrete form of a text delta is a well-formed XML element,
      having the following form:</p>

    <pre>
&lt;text-delta&gt;<em class="replaceable">data</em>&lt;/text-delta&gt;
</pre>

    <p>Here, <em class="replaceable">data</em> is the raw svndiff data,
      encoded in the MIME Base64 format.</p>
  </div> <!-- deltas.text (h3) -->

  <div class="h3" id="deltas.prop" title="#deltas.prop">
    <h3>Property Deltas</h3>
    
    
    <p>A property delta describes changes to a property list, of the sort
      associated with files, directories, and directory entries, and revision
      numbers (see <a href="#model.props">Properties</a>).  A property delta can record
      creating, deleting, and changing the text of any number of
      properties.</p>

    <p>A property delta is an unordered set of name/change pairs.  No two
      pairs within a given property delta have the same name.  A pair's name
      indicates the property affected, and the change indicates what happens to
      its value.  There are two kinds of changes:</p>

    <dl>
      <dt>set <em class="replaceable">value</em></dt>
        <dd><p>Change the value of the named property to the byte
            string <em class="replaceable">value</em>. If there is no property
            with the given name, one is added to the property
            list.</p></dd>
      
      <dt>delete</dt>
        <dd><p>Remove the named property from the property
            list.</p></dd>
      
    </dl>

    <p>At the moment, the <tt class="literal">set</tt> command can either create
      or change a property value.  However, this simplification means that the
      server cannot distinguish between a client which believes it is creating
      a value afresh, and a client which believes it is changing the value of
      an existing property.  It may simplify conflict detection to divide
      <tt class="literal">set</tt> into two separate <tt class="literal">add</tt> and
      <tt class="literal">change</tt> operations.</p>

    <p>In the future, we may add a <tt class="literal">text-delta</tt> change,
      which specifies a change to an existing property's value as a text delta.
      This would give us a compact way to describe small changes to large
      property values.</p>

    <p>The concrete form of a property delta is a well-formed XML element,
      having the following form:</p>

    <pre>
&lt;property-delta&gt;<em class="replaceable">change</em>&hellip;&lt;/property-delta&gt;
</pre>

    <p>Each <em class="replaceable">change</em> in a property delta has one of
      the following forms:</p>

    <pre>
&lt;set name='<em class="replaceable">name</em>'&gt;<em class="replaceable">value</em>&lt;/set&gt;
&lt;delete name='<em class="replaceable">name</em>'/&gt;
</pre>

    <p>The <em class="replaceable">name</em> attribute of a
      <tt class="literal">set</tt> or <tt class="literal">delete</tt> element gives the
      name of the property to change.  The <em class="replaceable">value</em> of
      a <tt class="literal">set</tt> element gives the new value of the
      property.</p>

    <p>If either the property name or the property value contains the
      characters &lsquo;<tt class="literal">&amp;</tt>&rsquo;,
      &lsquo;<tt class="literal">&lt;</tt>&rsquo;, or
      &lsquo;<tt class="literal">'</tt>&rsquo;, they should be replaced with the
      sequences &lsquo;<tt class="literal">&amp;#38</tt>&rsquo;,
      &lsquo;<tt class="literal">&amp;#60</tt>&rsquo;, or
      &lsquo;<tt class="literal">&amp;#39</tt>&rsquo;, respectively.</p>
  </div> <!-- deltas.prop (h3) -->

  <div class="h3" id="deltas.tree" title="#deltas.tree">
    <h3>Tree Deltas</h3>
    

    <p>A tree delta describes changes between two directory trees, the
      <strong class="firstterm">source tree</strong> and the <strong class="firstterm">target
        tree</strong>.  Tree deltas can describe copies, renames, and
      deletions of files and directories, changes to file contents, and changes
      to property lists.  A tree delta can also carry information about how the
      files in the target tree are derived from the files in the source tree,
      if this information is available.</p>

    <p>The format for tree deltas described here is easy to compute from a
      Subversion working directory, and easy to apply to a Subversion
      repository.  Furthermore, the size of a tree delta in this format is
      independent of the commands used to produce the target tree &mdash; it
      depends only on the degree of difference between the source and target
      trees.</p>

    <p>A tree delta is interpreted in the context of three
      parameters:</p>

    <ul>
      <li><p><em class="replaceable">source-root</em>, the name of the
          directory to which this complete tree delta applies,</p></li>
      <li><p><em class="replaceable">revision</em>, indicating a
          particular revision of &hellip;</p></li>
      <li><p><em class="replaceable">source-dir</em>, which is a
          directory in the source tree that we are currently modifying to yield
          &hellip;</p></li>
      <li><p>&hellip; <strong class="firstterm">target-dir</strong> &mdash; the
          directory we're constructing.</p></li>
    </ul>

    <p>When we start interpreting a tree delta,
      <em class="replaceable">source-root</em>,
      <em class="replaceable">source-dir</em>, and
      <em class="replaceable">target-dir</em> are all equal.  As we walk the tree
      delta, <em class="replaceable">target-dir</em> walks the tree we are
      constructing, and <em class="replaceable">source-dir</em> walks the
      corresponding portion of the source tree, which we use as the original.
      <em class="replaceable">Source-root</em> remains constant as we walk the
      delta; we may use it to choose new source trees.</p>

    <p>A tree delta is a list of changes of the form</p>

    <pre>
&lt;tree-delta&gt;<em class="replaceable">change</em>&hellip;&lt;/tree-delta&gt;
</pre>

    <p>which describe how to edit the contents of
      <em class="replaceable">source-dir</em> to yield
      <em class="replaceable">target-dir</em>.  There are three kinds of
      changes:</p>

    <dl>
      
        <dt>&lt;delete
          name='<em class="replaceable">name</em>'/&gt;</dt>
        <dd><p><em class="replaceable">Source-dir</em> has an entry
            named <em class="replaceable">name</em>, which is not present
            in <em class="replaceable">target-dir</em>.</p></dd>
      
      
        <dt>&lt;add
          name='<em class="replaceable">name</em>'&gt;<em class="replaceable">content</em>&lt;/add&gt;</dt>
        <dd><p><em class="replaceable">target-dir</em> has an entry
            named <em class="replaceable">name</em>, which is not present
            in <em class="replaceable">source-dir</em>;
            <em class="replaceable">content</em> describes the file or directory
            to which the new directory entry refers.</p></dd>
      
      
        <dt>&lt;open
          name='<em class="replaceable">name</em>'&gt;<em class="replaceable">content</em>&lt;/open&gt;</dt>
        <dd><p>Both <em class="replaceable">source-dir</em> and
            <em class="replaceable">target-dir</em> have an entry
            named <em class="replaceable">name</em>, which has changed;
            <em class="replaceable">content</em> describes the new file
            or directory.</p></dd>
      
    </dl>

    <p>Any entries in <em class="replaceable">source-dir</em> whose names
      aren't mentioned are assumed to appear unchanged in
      <em class="replaceable">target-dir</em>.  Thus, an empty
      <tt class="literal">tree-delta</tt> element indicates that
      <em class="replaceable">target-dir</em> is identical to
      <em class="replaceable">source-dir</em>.</p>

    <p>In the change descriptions above, each
      <em class="replaceable">content</em> takes one of the following
      forms:</p>

    <dl>
      
        <dt>&lt;file
          <em class="replaceable">ancestor</em>&gt;<em class="replaceable">prop-delta</em>
          <em class="replaceable">text-delta</em>&lt;/file&gt;</dt>

        <dd><p>The given <em class="replaceable">target-dir</em> entry
            refers to a file, <em class="replaceable">f</em>.
            <em class="replaceable">Ancestor</em> indicates which file in the
            source tree <em class="replaceable">f</em> is derived from, if any.
          </p>

          <p><em class="replaceable">Prop-delta</em> is a property delta
            describing how <em class="replaceable">f</em>'s properties differ
            from that ancestor; it may be omitted, indicating that the
            properties are unchanged.</p>
          
          <p><em class="replaceable">Text-delta</em> is a text delta
            describing how to construct <em class="replaceable">f</em> from that
            ancestor; it may also be omitted, indicating that
            <em class="replaceable">f</em>'s text is identical to its
            ancestor's.</p></dd>
      

      
        <dt>&lt;file <em class="replaceable">ancestor</em>/&gt;</dt>

        <dd><p>An abbreviation for <tt class="literal">&lt;file
              <em class="replaceable">ancestor</em>&gt;&lt;/file&gt;</tt>
            &mdash; a fileelement with no property or text delta, thus
            describing a file identicalto its ancestor.</p></dd>
      

      
        <dt>&lt;directory
          <em class="replaceable">ancestor</em>&gt;<em class="replaceable">prop-delta</em>
          <em class="replaceable">tree-delta</em>&lt;/directory&gt;</dt>

        <dd><p>The given <em class="replaceable">target-dir</em> entry
            refers to a subdirectory, <em class="replaceable">sub</em>.
            <em class="replaceable">Ancestor</em> indicates which directory in
            the source tree <em class="replaceable">sub</em> is derived from, if
            any.</p>
            
          <p><em class="replaceable">Prop-delta</em> is a property delta
            describing how <em class="replaceable">sub</em>'sproperties differ
            from that ancestor; it may be omitted, indicating thatthe
            properties are unchanged.</p>
            
          <p><em class="replaceable">Tree-delta</em>
            describes how to construct <em class="replaceable">sub</em> from
            that ancestor; it may be omitted, indicating that the directory is
            identical to its ancestor.  <em class="replaceable">Tree-delta</em>
            should be interpreted with a new
            <em class="replaceable">target-dir</em> of
            <tt class="filename"><em class="replaceable">target-dir</em>/<em class="replaceable">name</em></tt>.</p>
            
          <p>Since <em class="replaceable">tree-delta</em> is itself a
            complete tree delta structure, tree deltas are themselves trees,
            whose structure is a subgraph of the target tree.</p></dd>
      

      
        <dt>&lt;directory
          <em class="replaceable">ancestor</em>/&gt;</dt>

        <dd><p>An abbreviation for <tt class="literal">&lt;directory
              <em class="replaceable">ancestor</em>&gt;&lt;/directory&gt;</tt>
            &mdash; a directory element with no property or tree delta, thus
            describing a directory identical to its ancestor.</p></dd>
      
    </dl>

    <p>The <em class="replaceable">content</em> of a <tt class="literal">add</tt> or
      <tt class="literal">open</tt> tag may also contain a property delta, describing
      changes to the properties of that <em>directory
        entry</em>.</p>

    <p>In the <tt class="literal">file</tt> and <tt class="literal">directory</tt>
      elements described above, each <em class="replaceable">ancestor</em> has
      one of the following forms:</p>

    <dl>
      
        <dt>ancestor='<em class="replaceable">path</em>'</dt>

        <dd><p>The ancestor of the new or changed file or directory is
            <tt class="filename"><em class="replaceable">source-root</em>/<em class="replaceable">path</em></tt>,
            in <em class="replaceable">revision</em>.  When this appears as an
            attribute of a <tt class="literal">file</tt> element, the element's text
            delta should be applied to
            <tt class="filename"><em class="replaceable">source-root</em>/<em class="replaceable">path</em></tt>.
            When this appears as an attribute of a <tt class="literal">directory</tt>
            element,
            <tt class="filename"><em class="replaceable">source-root</em>/<em class="replaceable">path</em></tt>
            should be the new <em class="replaceable">source-dir</em> for
            interpreting that element's tree delta.</p></dd>
      

      
        <dt>new='true'</dt>

        <dd><p>This indicates that the file or directory has no
            ancestor in the source tree.  When followed by a
            <em class="replaceable">text-delta</em>, that delta should be applied
            to the empty file to yield the new text; when followed by a
            <em class="replaceable">tree-delta</em>, that delta should be
            evaluated as if <em class="replaceable">source-dir</em> were an
            imaginary empty directory.</p></dd>
      

      
        <dt><em class="replaceable">nothing</em></dt>

        <dd><p>If neither an <tt class="literal">ancestor</tt> nor a
            <tt class="literal">new</tt> attribute is given, this is an abbreviation
            for
            <tt class="literal">ancestor='<em class="replaceable">source-dir</em>/<em class="replaceable">name</em>'</tt>,
            with the same revision number.  This makes the common case &mdash;
            files or directories modified in place &mdash; more
            compact.</p></dd>
      
    </dl>

    <p>If the <em class="replaceable">ancestor</em> spec is not
      <tt class="literal">new='true'</tt>, it may also contain the text
      <tt class="literal">revision='<em class="replaceable">rev</em>'</tt>, indicating
      a new value for <em class="replaceable">revision</em>, in which we should
      find the ancestor.</p>

    <p>If a filename or path appearing as a <em class="replaceable">name</em>
      or <em class="replaceable">path</em> in the description above contains the
      characters &lsquo;<tt class="literal">&amp;</tt>&rsquo;,
      &lsquo;<tt class="literal">&lt;</tt>&rsquo;, or
      &lsquo;<tt class="literal">'</tt>&rsquo;, they should be replaced with the
      sequences &lsquo;<tt class="literal">&amp;#38;</tt>&rsquo;,
      &lsquo;<tt class="literal">&amp;#60;</tt>&rsquo;, or
      &lsquo;<tt class="literal">&amp;#39;</tt>&rsquo;, respectively.</p>

    <p>Suppose we have the following source tree:</p>

    <pre>
/dir1/file1
      file2
      dir2/file3
           file4
      dir3/file5
           file6
</pre>

    <p>If we edit the contents of <tt class="filename">/dir1/file1</tt>, we can
      describe the effect on the tree with the following tree delta, to be
      applied to the root:</p>

    <pre>
&lt;tree-delta&gt;
  &lt;open name='dir1'&gt;
    &lt;directory&gt;
      &lt;tree-delta&gt;
        &lt;open name='file1'&gt;
          &lt;file&gt;<em class="replaceable">text-delta</em>&lt;/file&gt;
        &lt;/open&gt;
      &lt;/tree-delta&gt;
    &lt;/directory&gt;
  &lt;/open&gt;
&lt;/tree-delta&gt;
</pre>

    <p>The outer <tt class="literal">tree-delta</tt> element describes the changes
      made to the root directory.  Within the root directory, there are changes
      in <tt class="filename">dir1</tt>, described by the nested
      <tt class="literal">tree-delta</tt>.  Within <tt class="filename">/dir1</tt>, there
      are changes in <tt class="filename">file1</tt>, described by the
      <em class="replaceable">text-delta</em>.</p>

    <p>If we had edited both <tt class="filename">/dir1/file1</tt> and
      <tt class="filename">/dir1/file2</tt>, then there would simply be two
      <tt class="literal">open</tt> elements in the inner
      <tt class="literal">tree-delta</tt>.</p>

    <p>As another example, starting from the same source tree, suppose we
      rename <tt class="filename">/dir1/file1</tt> to
      <tt class="filename">/dir1/file8</tt>:</p>

    <pre>
&lt;tree-delta&gt;
  &lt;open name='dir1'&gt;
    &lt;directory&gt;
      &lt;tree-delta&gt;
        &lt;delete name='file1'/&gt;
        &lt;add name='file8'&gt;
          &lt;file ancestor='/dir1/file1'/&gt;
        &lt;/add&gt;
      &lt;/tree-delta&gt;
    &lt;/directory&gt;
  &lt;/open&gt;
&lt;/tree-delta&gt;
</pre>

    <p>As above, the inner <tt class="literal">tdelta</tt> describes how
      <tt class="filename">/dir1</tt> has changed: the entry for
      <tt class="filename">/dir1/file1</tt> has disappeared, but there is a new
      entry, <tt class="filename">/dir1/file8</tt>, which is derived from and
      textually identical to <tt class="filename">/dir1/file1</tt> in the source
      directory.  This is just an indirect way of describing the rename.</p>

    <p>Why is it necessary to be so indirect?  Consider the delta
      representing the result of:</p>

    <ol>
      <li><p>renaming <tt class="filename">/dir1/file1</tt> to
          <tt class="filename">/dir1/tmp</tt>,</p></li>
      <li><p>renaming <tt class="filename">/dir1/file2</tt> to
          <tt class="filename">/dir1/file1</tt>, and</p></li>
      <li><p>renaming <tt class="filename">/dir1/tmp</tt> to
          <tt class="filename">/dir1/file2</tt></p></li>
    </ol>

    <p>(in other words, exchanging <tt class="filename">file1</tt> and
      <tt class="filename">file2</tt>):</p>

    <pre>
&lt;tree-delta&gt;
  &lt;open name='dir1'&gt;
    &lt;directory&gt;
      &lt;tree-delta&gt;
        &lt;open name='file1'&gt;
          &lt;file ancestor='/dir1/file2'/&gt;
        &lt;/open&gt;
        &lt;open name='file2'&gt;
          &lt;file ancestor='/dir1/file1'/&gt;
        &lt;/open&gt;
      &lt;/tree-delta&gt;
    &lt;/directory&gt;
  &lt;/open&gt;
&lt;/tree-delta&gt;
</pre>

    <p>The indirectness allows the tree delta to capture an arbitrary
      rearrangement without resorting to temporary filenames.</p>

    <p>Another example, starting from the same source tree:</p>

    <ol>
      <li><p>rename <tt class="filename">/dir1/dir2</tt> to
          <tt class="filename">/dir1/dir4</tt>,</p></li>
      <li><p>rename <tt class="filename">/dir1/dir3</tt> to
          <tt class="filename">/dir1/dir2</tt>, and</p></li>
      <li><p>move <tt class="filename">file3</tt> from
          <em class="replaceable">/dir1/dir4</em> to
          <em class="replaceable">/dir1/dir2</em>.</p></li>
    </ol>

    <p>Note that <tt class="filename">file3</tt>'s path has remained the same,
      even though the directories around it have changed.  Here is the tree
      delta:</p>

    <pre>
&lt;tree-delta&gt;
  &lt;open name='dir1'&gt;
    &lt;directory&gt;
      &lt;tree-delta&gt;
        &lt;open name='dir2'&gt;
          &lt;directory ancestor='/dir1/dir3'&gt;
            &lt;tree-delta&gt;
              &lt;add name='file3'&gt;
                &lt;file ancestor='/dir1/dir2/file3'/&gt;
              &lt;/add&gt;
            &lt;/tree-delta&gt;
          &lt;/directory&gt;
        &lt;/open&gt;
        &lt;delete name='dir3'/&gt;
        &lt;add name='dir4'&gt;
          &lt;directory ancestor='/dir1/dir2'&gt;
            &lt;tree-delta&gt;
              &lt;delete name='file3'/&gt;
            &lt;/tree-delta&gt;
          &lt;/directory&gt;
        &lt;/add&gt;
      &lt;/tree-delta&gt;
    &lt;/directory&gt;
  &lt;/open&gt;
&lt;/tree-delta&gt;
</pre>

    <p>In other words:</p>

    <ul>
      <li><p><tt class="filename">/dir1</tt> has changed;</p></li>
      <li><p>the new directory <tt class="filename">/dir1/dir2</tt> is
          derived from the old <tt class="filename">/dir1/dir3</tt>, and contains a
          new entry <tt class="filename">file3</tt>, derived from the old
          <tt class="filename">/dir1/dir2/file3</tt>;</p></li>
      <li><p>there is no longer any <tt class="filename">/dir1/dir3</tt>;
          and</p></li>
      <li><p>the new directory <tt class="filename">/dir1/dir4</tt> is
          derived from the old <tt class="filename">/dir1/dir2</tt>, except that its
          entry for <tt class="filename">file3</tt> is now gone.</p></li>
    
    </ul>
    
    <p>Some more possible maneuvers, left as exercises for the
      reader:</p>

    <ul>
      <li><p>Delete <tt class="filename">dir2</tt>, and then create a file
          named <tt class="filename">dir2</tt>.</p></li>
      <li><p>Rename <tt class="filename">/dir1/dir2</tt> to
          <tt class="filename">/dir1/dir4</tt>; move <tt class="filename">file2</tt>
          into <tt class="filename">/dir1/dir4</tt>; and move
          <tt class="filename">file3</tt> into
          <em class="replaceable">/dir1/dir3</em>.</p></li>
      <li><p>Move <tt class="filename">dir2</tt> into
          <tt class="filename">dir3</tt>, and move <tt class="filename">dir3</tt> into
          <tt class="filename">/</tt>.</p></li>
    </ul>
  </div> <!-- deltas.tree (h3) -->

  <div class="h3" id="deltas.postfix-text" title="#deltas.postfix-text">
    <h3>Postfix Text Deltas</h3>
    

    <p>It is sometimes useful to represent a set of changes to a tree
      without providing text deltas in the middle of the stream.  Text deltas
      are often large and expensive to compute, and tree deltas can be useful
      without them.  For example, one can detect whether two changes might
      conflict &mdash; whether they change the same file, for example &mdash;
      without knowing exactly how the conflicting files changed.</p>

    <p>For this reason, our XML representation of a tree delta allows the
      text deltas to come <em>after</em> the &lt;/tree-delta&gt;
      closure.  This allows the client to receive early notice of conflicts:
      during a <tt class="literal">svn commit</tt> command, the client sends a
      tree-delta to the server, which can check for skeletal conflicts and
      reject the commit, before the client takes the time to transmit the
      (possibly large) textual changes.  This potentially saves quite a bit of
      network traffic.</p>

    <p>In terms of XML, postfix text deltas are split into two parts.  The
      first part appears "in-line" and contains a reference ID.  The second
      part appears after the tree delta is complete.  Here's an example:</p>

    <pre>
 &lt;tree-delta&gt;
   &lt;open name="foo.c"&gt;
      &lt;file&gt;
        &lt;text-delta-ref id="123"&gt;
      &lt;/file&gt;
   &lt;/open&gt;
   &lt;add name="bar.c"&gt;
      &lt;file&gt;
        &lt;text-delta-ref id="456"&gt;
      &lt;/file&gt;
    &lt;/add&gt;
 &lt;/tree-delta&gt;
 &lt;text-delta id="123"&gt;<em>data</em>&lt;/text-delta&gt;
 &lt;text-delta id="456"&gt;<em>data</em>&lt;/text-delta&gt;
</pre>

  </div> <!-- deltas.postfix-text (h3) -->

  <div class="h3" id="deltas.serializing-via-editor" title="#deltas.serializing-via-editor">
    <h3>Serializing Deltas via the "Editor" Interface</h3>
    

    <p>The static XML forms above are useful as an import/export format, and
      as a visualization aid, but we also need a way to express a delta as a
      <em>series of operations</em>, to implement directory tree
      diffing and patching.  Subversion defines a standard set of such
      operations in the vtable <tt class="literal">svn_delta_edit_fns_t</tt>, a set
      of function prototypes which anyone may implement (see
      <tt class="filename">svn_delta.h</tt>).</p>

    <p>Each function in an instance of <tt class="literal">svn_delta_editor_t</tt>
      (colloquially known as an <strong class="firstterm">editor</strong>) implements some
      distinct subtask of editing a directory tree.  In fact, if you compare
      the editor function prototypes to the XML elements described previously,
      you'll notice a fairly strict correspondence: there's one function for
      replacing a directory, another function for replacing a file, one for
      adding a directory, another for adding a file, a function for deleting,
      and so on.</p>

    <p>Although the editor interface was designed around the general idea of
      making changes to a directory tree, a specific implementation's behavior
      depends on its role.  For example, the versioning filesystem library
      offers an editor that creates new revisions, while the working copy
      library offers an editor that updates working copies.  And the network
      layer offers an editor that turns editing calls into wire protocol, which
      is then converted back into editing calls on the other side!  All of
      these different tasks can share a single interface, because they are all
      fundamentally about the same thing: expressing and applying differences
      between directory trees.</p>

    <p>Like the XML forms, a series of editor calls must follow certain
      nesting conventions; these conventions are implicit in the interface, in
      that some of the functions take arguments that can only be obtained from
      previous calls to other editor functions.</p>

    <p>Editors can best be understood by watching one work on a real
      directory tree.  For example:</p>

<!-- kff todo: fooo working here. -->

    <p>Suppose that the user has made a number of local changes to her
      working copy and wants to commit them to the repository.  Let's represent
      her changes with the same tree-delta from a previous example.  Notice
      that she has also made textual modifications to
      <tt class="filename">file3</tt>; hence the in-line
      <tt class="literal">&lt;text-delta&gt;</tt>:</p>

    <pre>
&lt;tree-delta&gt;
  &lt;open name='dir1'&gt;
    &lt;directory&gt;
      &lt;tree-delta&gt;
        &lt;open name='dir2'&gt;
          &lt;directory ancestor='/dir1/dir3'&gt;
            &lt;tree-delta&gt;
              &lt;add name='file3'&gt;
                &lt;file ancestor='/dir1/dir2/file3'&gt;
                  &lt;text-delta&gt;<em>data</em>&lt;/text-delta&gt;
                &lt;/file&gt;
              &lt;/add&gt;
            &lt;/tree-delta&gt;
          &lt;/directory&gt;
        &lt;/open&gt;
        &lt;delete name='dir3'/&gt;
        &lt;add name='dir4'&gt;
          &lt;directory ancestor='/dir1/dir2'&gt;
            &lt;tree-delta&gt;
              &lt;delete name='file3'/&gt;
            &lt;/tree-delta&gt;
          &lt;/directory&gt;
        &lt;/add&gt;
      &lt;/tree-delta&gt;
    &lt;/directory&gt;
  &lt;/open&gt;
&lt;/tree-delta&gt;
</pre>

    <p>So how does the client send this information to the server?</p>

    <p>In a nutshell: the tree-delta is <em>streamed</em> over
      the network, as a series of individual commands given in depth-first
      order.</p>

    <p>Let's be more specific.  The server presents the client with an
      object of type <tt class="literal">struct svn_delta_edit_fns_t</tt>,
      colloquially known as an <strong class="firstterm">editor</strong>.  An editor is
      really just table of functions; each function makes a change to a
      filesystem.  Agent A (who has a private filesystem) presents an editor to
      agent B.  Agent B then calls the editor's functions to change A's
      filesystem.  B is said to be <strong class="firstterm">driving</strong> the
      editor.</p>

    <p>As Karl Fogel likes to describe the process, if one thinks of the
      tree-delta as a lion, the editor is a "hoop" that the lion jumps through
      &ndash; each portion of the lion being decomposed through time.</p>

    <p>B cannot call the functions in any willy-nilly order; there are some
      logical restrictions.  In particular, as B drives the editor, it receives
      opaque data structures which represent directories and files.  It must
      use and pass these structures, known as <strong class="firstterm">batons</strong>, to
      make further function calls.</p>

    <p>As an example, let's watch how the client would transmit the above
      tree-delta to the repository.  (The description below is slightly
      simplified.  For exact interface details, see
      <tt class="filename">subversion/include/svn_delta.h</tt>.)</p>

    <p>[Note:  in the examples below, and throughout Subversion's code base,
      you'll see references to 'baton' objects.  This is simply a project
      convention, a name given to structures that define contexts for
      functions.  Many APIs call these structures 'userdata'.  In Subversion,
      we like the term 'baton', because it reminds us of one function
      &ldquo;handing off&rdquo; context to another function.]</p>

    <ol>
      <li><p>The repository hands an "editor" to the
          client.</p></li>

      <li><p>The client begins by calling <tt class="literal">root_baton =
            editor-&gt;open_root();</tt> The client now has an opaque
          object, <strong class="firstterm">root_baton</strong>, which represents the root
          of the repository's filesystem.</p></li>

      <li><p><tt class="literal">dir1_baton = editor-&gt;open_dir("dir1",
            root_baton);</tt> Notice that <em>root_baton</em>
          gives the client free license to make any changes it wants in the
          repository's root directory &ndash; until, of course, it calls
          <tt class="literal">editor-&gt;close_dir(root_baton)</tt>.  The first
          change made was a replacement of <tt class="filename">dir1</tt>.  In
          return, the client now has a new opaque data structure that can be
          used to change <tt class="filename">dir1</tt>.</p></li>

      <li><p><tt class="literal">dir2_baton = editor-&gt;open_dir("dir2",
            "/dir1/dir3", dir1_baton);</tt> The
          <em>dir1_baton</em> is now used to open
          <tt class="filename">dir2</tt> with a directory whose ancestor is
          <tt class="filename">/dir1/dir3</tt>.</p></li>

      <li><p><tt class="literal">file_baton = editor-&gt;add_file("file3",
            "/dir1/dir2/file3", dir2_baton);</tt> Edits are now made to
          <tt class="filename">dir2</tt> (using <em>dir2_baton</em>).
          In particular, a new file is added to this directory whose ancestor
          is <tt class="filename">/dir1/dir2/file3</tt>.</p></li>

      <li><p>Now the text-delta associated with
          <em>file_baton</em> needs to be transmitted:
          <tt class="literal">window_handler =
            editor-&gt;apply_textdelta(file_baton);</tt> Text-deltas
          themselves, for network efficiency, are streamed in "chunks".  So
          instead of receiving a baton object, we now have a routine that is
          able to receive any number of small "windows" of text-delta data.We
          won't go into the details of the <tt class="literal">svn_txdelta_*</tt>
          functions right here;  but suffice it to say that these routines are
          used for sending svndiff data to the
          <em>window_handler</em> routine.</p></li>

      <li><p><tt class="literal">editor-&gt;close_file(file_baton);</tt> The
          client is done sending the file's text-delta, so it releases the file
          baton.</p></li>

      <li><p><tt class="literal">editor-&gt;close_dir(dir2_baton));</tt> The
          client is done making changes to <tt class="filename">dir2</tt>, so it
          releases its baton as well.</p></li>

      <li><p>The client isn't yet finished with
          <tt class="filename">dir1</tt>, however; it makes two more edits:
          <tt class="literal">editor-&gt;delete_item("dir3", dir1_baton);</tt>
          <tt class="literal">dir4_baton = editor-&gt;add_dir("dir4", "/dir1/dir2",
            dir1_baton);</tt> <em>(The function's name is
            <tt class="literal">delete_item</tt> rather than
            <tt class="literal">delete</tt> to avoid gratuitous incompatibility with
            C++, where <tt class="literal">delete</tt> is a reserved
            keyword.)</em></p></li>

      <li><p>Within the directory <tt class="filename">dir4</tt> (whose
          ancestry is <tt class="filename">/dir1/dir2</tt>), the client removes a
          file:  <tt class="literal">editor-&gt;delete_item("file3",
            dir4_baton);</tt></p></li>

      <li><p>The client is now finished with both
          <tt class="filename">dir4</tt>, as well as its
          parent <tt class="filename">dir1</tt>:
          <tt class="literal">editor-&gt;close_dir(dir4_baton);</tt>
          <tt class="literal">editor-&gt;close_dir(dir1_baton);</tt></p></li>

      <li><p>The entire tree-delta is complete.  The repository knows
          this when the root directory is closed:
          <tt class="literal">editor-&gt;close_dir(root_baton);</tt></p></li>

    </ol>
    
    <p>Of course, at any point above, the repository may reject an edit.  If
      this is the case, the client aborts the transmission and the repository
      hasn't changed a bit.  (Thank goodness for transactions!)</p>

    <p>Note, however, that this "editor interface" works in the other
      direction as well.  When the repository wishes to update a client's
      working copy, it is the <em>client's</em> reponsibility to
      give a custom editor-object to the server, and the
      <em>server</em> is the editor-driver.</p>

    <p>Here are the main advantages of this interface:</p>

    <ul>
      <li><p><em>Consistency</em>.  Tree-deltas move
          across the network, in both directions, using the same
          interface.</p></li>
      <li><p><em>Flexibility</em>.  Custom
          editor-implementations can be written to do anything one might want;
          the editor-driver has no idea what is happening on the other side of
          the interface.  For example, an editor might
          </p><ul>
            <li><p>Output XML that matches the tree-delta DTD
                above;</p></li>
            <li><p>Output human-readable descriptions of the edits
                taking place;</p></li>
            <li><p>Modify a filesystem</p></li>
          </ul><p>
      </p></li>
    </ul>

    <p>Whatever the case, it's easy to "swap" editors around, and make
      client and server do new and interesting things.</p>
  </div> <!-- deltas.serializing-via-editor (h3) -->
</div> <!-- deltas (h2) -->

  <div class="h2" id="client" title="#client">
  <h2>Client &mdash; How the client works</h2>
  

  
    <p>The Subversion client is built on three libraries.  One operates
      strictly on the working copy and does not talk to the repository.
      Another talks to the repository but never changes the working copy.  The
      third library uses the first two to provide operations such as
      <tt class="literal">commit</tt> and <tt class="literal">update</tt> &ndash;
      operations which need to both talk to the repository and change the
      working copy.</p>

    <p>The initial client is a Unix-style command-line tool (like standard
      CVS), but it should be easy to write a GUI client as well, based on the
      same libraries.  The libraries capture the core Subversion functionality,
      segregating it from user interface concerns.</p>

    <p>This chapter describes the libraries, and the physical layout of
      working copies.</p>
  

  <div class="h3" id="client.wc" title="#client.wc">
    <h3>Working copies and the working copy library</h3>
    

    <p>Working copies are client-side directory trees containing both
      versioned data and Subversion administrative files.  The functions in the
      working copy management library are the only functions in Subversion
      which operate on these trees.</p>

    <div class="h4" id="client.wc.layout" title="#client.wc.layout">
      <h4>The layout of working copies</h4>
      

      <p>This section gives an overview of how
        working copies are arranged physically, but is not a full specification
        of working copy layout.</p>

      <p>As with CVS, Subversion working copies are simply directory trees
        with special administrative subdirectories, in this case named ".svn"
        instead of "CVS":</p>

      <pre>
                             myproj
                             / | \
               _____________/  |  \______________
              /                |                 \
           .svn               src                doc
        ___/ | \___           /|\             ___/ \___
       |     |     |         / | \           |         |
      base  ...   ...       /  |  \     myproj.texi  .svn
                           /   |   \              ___/ | \___
                      ____/    |    \____        |     |     |
                     |         |         |      base  ...   ...
                   .svn      foo.c     bar.c     |
                ___/ | \___                      |
               |     |     |                     |
             base   ...   ...               myproj.texi
          ___/ \___
         |         |
       foo.c     bar.c

</pre>

      <p>Each <tt class="filename">dir/.svn/</tt> directory records the files in
        <tt class="filename">dir</tt>, their revision numbers and property lists,
        pristine revisions of all the files (for client-side delta generation),
        the repository from which <tt class="filename">dir</tt> came, and any local
        changes (such as uncommitted adds, deletes, and renames) that affect
        <tt class="filename">dir</tt>.</p>

      <p>Although it would often be possible to deduce certain information
        (such as the original repository) by examining parent directories, this
        is avoided in favor of making each directory be as much a
        self-contained unit as possible.</p>

      <p>For example, immediately after a checkout the administrative
        information for the entire working tree <em>could</em> be
        stored in one top-level file.  But subdirectories instead keep track of
        their own revision information.  This would be necessary anyway once
        the user starts committing new revisions for particular files, and it
        also makes it easier for the user to prune a big, complete tree into a
        small subtree and still have a valid working copy.</p>

      <p>The <tt class="filename">.svn</tt> subdir contains:</p>

      <ul>
          <li><p>A <tt class="filename">format</tt> file, which indicates
              which version of the working copy adm format this is (so future
              clients can be backwards compatible easily).</p></li>

          <li><p>A <tt class="filename">text-base</tt> directory,
              containing the pristine repository revisions of the files in the
              corresponding working directory</p></li>

          <li><p>An <tt class="filename">entries</tt> file, which holds
              revision numbers and other information for this directory and its
              files, and records the presence of subdirs.  It also contains the
              repository URLs that each file and directory came from. It may
              help to think of this file as the functional equivalent of the
              <tt class="filename">CVS/Entries</tt> file.</p></li>

          <li><p>A <tt class="filename">props</tt> directory, containing
              property names and values for each file in the working
              directory.</p></li>

          <li><p>A <tt class="filename">prop-base</tt> directory,
              containing pristine property names and values for each file in
              the working directory.</p></li>

          <li><p>A <tt class="filename">dir-props</tt> file, recording
              properties for this directory.</p></li>

          <li><p>A <tt class="filename">dir-prop-base</tt> file, recording
              pristine properties for this directory.</p></li>

          <li><p>A <tt class="filename">lock</tt> file, whose presence
              implies that some client is currently operating on the
              administrative area.</p></li>

          <li><p>A <tt class="filename">tmp</tt> directory, for holding
              scratch-work and helping make working copy operations more
              crash-proof.</p></li>

          <li><p>A <tt class="filename">log</tt> file.  If present,
              indicates a list of actions that need to be taken to complete a
              working-copy-operation that is still "in
              progress".</p></li>
        </ul>

      <p>You can read much more about these files in the file
        <tt class="filename">subversion/libsvn_wc/README</tt>.</p>
    </div> <!-- client.wc.layout (h4) -->

    <div class="h4" id="client.wc.library" title="#client.wc.library">
      <h4>The working copy management library</h4>
      

      <ul>
        <li><p><strong>Requires:</strong>  
          </p><ul>
            <li><p>a working copy</p></li>
          </ul><p>
        </p></li>
        <li><p><strong>Provides:</strong>  
          </p><ul>
            <li><p>ability to manipulate the working copy's versioned
                data</p></li>
            <li><p>ability to manipulate the working copy's
                administrative files</p></li>
          </ul><p>
        </p></li>
      </ul>

      <p>This library performs "offline" operations on the working copy, and
        lives in <tt class="filename">subversion/libsvn_wc/</tt>.</p>

      <p>The API for <em class="replaceable">libsvn_wc</em> is always
        evolving;  please read the header file for a detailed description:
        <tt class="filename">subversion/include/svn_wc.h</tt>.</p>
    </div> <!-- client.wc.library (h4) -->
  </div> <!-- client.wc (h3) -->

  <div class="h3" id="client.libsvn_ra" title="#client.libsvn_ra">
    <h3>The repository access library</h3>
    

    <ul>
      <li><p><strong>Requires:</strong>  
        </p><ul>
          <li><p>network access to a Subversion
              server</p></li>
        </ul><p>
      </p></li>
      <li><p><strong>Provides:</strong>  
        </p><ul>
          <li><p>the ability to interact with a
              repository</p></li>
        </ul><p>
      </p></li>
    </ul>

    <p>This library performs operations involving communication with the
      repository.</p>

    <p>The interface defined in
      <tt class="filename">subversion/include/svn_ra.h</tt> provides a uniform
      interface to both local and remote repository access.</p>

    <p>Specifically, <em class="replaceable">libsvn_ra_dav</em> will provide
      this interface and speak to repositories using DAV requests.  At some
      future point, another library <em class="replaceable">libsvn_ra_local</em>
      will provide the same interface &ndash; but will link directly to the
      filesystem library for accessing local disk repositories.</p>
  </div> <!-- client.libsvn_ra (h3) -->

  <div class="h3" id="client.libsvn_client" title="#client.libsvn_client">
    <h3>The client operation library</h3>
    
    
    <ul>
      <li><p><strong>Requires:</strong>  
        </p><ul>
          <li><p>the working copy management library</p></li>
          <li><p>a repository access library</p></li>
        </ul><p>
      </p></li>
      <li><p><strong>Provides:</strong>  
        </p><ul>
          <li><p>all client-side Subversion commands</p></li>
        </ul><p>
      </p></li>
    </ul>

    <p>These functions correspond to user-level client commands.  In theory,
      any client interface (command-line, GUI, emacs, Python, etc.) should be
      able to link to <em class="replaceable">libsvn_client</em> and have the
      ability to act as a full-featured Subversion client.</p>

    <p>Again, the detailed API can be found in
      <tt class="filename">subversion/include/svn_client.h</tt>.</p>
  </div> <!-- client.libsvn_client (h3) -->
</div> <!-- client (h2) -->

  <div class="h2" id="protocol" title="#protocol">
  <h2>Protocol &mdash; How the client and server communicate</h2>
  

  
    <p>The wire protocol is the connection between the servers, and the
      client-side <em>Repository Access (RA) API</em>, provided by
      <tt class="literal">libsvn_ra</tt>.  Note that <tt class="literal">libsvn_ra</tt> is
      in fact only a plugin manager, which delegates the actual task of
      communicating with a server to one of a selection of back-end modules (the
      <tt class="literal">libsvn_ra_*</tt> libraries).  Therefore, there is not just
      one Subversion protocol - in fact, at present, there are two:</p>
    
    <ul>
      <li><p>The HTTP/WebDAV/DeltaV based protocol, implemented by the
          <tt class="literal">mod_dav_svn</tt> Apache 2 server module, and by two
          independent RA modules, <tt class="literal">libsvn_ra_dav</tt> and
          <tt class="literal">libsvn_ra_serf</tt>.</p></li>

      <li><p>The custom-designed protocol built directly upon TCP,
          implemented by the <tt class="literal">svnserve</tt> server, and the
          <tt class="literal">libsvn_ra_svn</tt> RA module.</p></li>
    </ul>
  

  <div class="h3" id="protocol.webdav" title="#protocol.webdav">
    <h3>The HTTP/WebDAV/DeltaV based protocol</h3>
    

    <p>The Subversion client library <tt class="literal">libsvn_ra_dav</tt> uses
      the <em>Neon</em> library to generate WebDAV DeltaV requests
      and sends them to a "Subversion-aware" Apache server.</p>

    <p>This Apache server is running <tt class="literal">mod_dav</tt> and
      <tt class="literal">mod_dav_svn</tt>, which translates the requests into
      Subversion filesystem calls.</p>

    <p>For more info, see <a href="#archi.network">Network Layer</a>.</p>

    <p>For a detailed description of exactly how Greg Stein
      <em class="email">gstein@lyra.org</em> is mapping the WebDAV DeltaV spec to
      Subversion, see his paper: <a href="http://svn.collab.net/repos/svn/trunk/www/webdav-usage.html">http://svn.collab.net/repos/svn/trunk/www/webdav-usage.html</a>
    </p>

    <p>For more information on WebDAV and the DeltaV extensions, see
      <a href="http://www.webdav.org">http://www.webdav.org</a> and
      <a href="http://www.webdav.org/deltav">http://www.webdav.org/deltav</a>.
    </p>

    <p>For more information on <em>Neon</em>, see
      <a href="http://www.webdav.org/neon">http://www.webdav.org/neon</a>.</p>
  </div> <!-- protocol.webdav (h3) -->

  <div class="h3" id="protocol.svn" title="#protocol.svn">
    <h3>The custom protocol</h3>
    

    <p>The client library <tt class="literal">libsvn_ra_svn</tt> and standalone
      server program <tt class="literal">svnserve</tt> implement a custom protocol
      over TCP.  This protocol is documented at <a href="http://svn.collab.net/repos/svn/trunk/subversion/libsvn_ra_svn/protocol">http://svn.collab.net/repos/svn/trunk/subversion/libsvn_ra_svn/protocol</a>.</p>
  </div> <!-- protocol.svn (h3) -->
</div> <!-- protocol (h2) -->

  <div class="h2" id="server" title="#server">
  <h2>Server &mdash; How the server works</h2>
  

  
    <p>The term &ldquo;server&rdquo; is ambiguous, because it has at least
      two different meanings: it can refer to a powerful computer which offers
      services to users on a network, or it can refer to a CPU process designed
      to receive network requests.</p>

    <p>In Subversion, however, the <strong class="firstterm">server</strong> is just a
      set of libraries that implements <strong class="firstterm">repositories</strong> and
      makes them available to other programs.  No networking is
      required.</p>

    <p>There are two main libraries: the <strong class="firstterm">Subversion
        Filesystem</strong> library, and the <strong class="firstterm">Subversion
        Repository</strong> library.</p>
  

  <div class="h3" id="server.fs" title="#server.fs">
    <h3>Filesystem</h3>
    

    <div class="h4" id="server.fs.overview" title="#server.fs.overview">
      <h4>Filesystem Overview</h4>
      
      <ul>
        <li><p><strong>Requires:</strong>  
          </p><ul>
            <li><p>some writable disk space</p></li>
            <li><p>(for now) Berkeley DB library</p></li>
          </ul><p>
        </p></li>
        <li><p><strong>Provides:</strong>  
          </p><ul>
            <li><p>a repository for storing files</p></li>
            <li><p>concurrent client transactions</p></li>
            <li><p>enforcement of user &amp; group permissions
                [someday, not yet]</p></li>
          </ul><p>
        </p></li>
      </ul>
      <p>This library implements a hierarchical filesystem which supports
        atomic changes to directory trees, and records a complete history of
        the changes.  In addition to recording changes to file and directory
        contents, the Subversion Filesystem records changes to file meta-data
        (see discussion of <strong class="firstterm">properties</strong> in <a href="#model">Model &mdash; The versioning model used by Subversion</a>).</p>
    </div> <!-- server.fs.overview (h4) -->

    <div class="h4" id="server.fs.api" title="#server.fs.api">
      <h4>API</h4>
      

      <p> There are two main files that describe the Subversion
        filesystem.</p>

      <p>First, read the section below (<a href="#server.fs.struct">Repository Structure</a>)
        for a general overview of how the filesystem works.</p>

      <p>Once you've done this, read Jim Blandy's own structural overview,
        which explains how nodes and revisions are organized (among other
        things) in the filesystem implementation:
        <tt class="filename">subversion/libsvn_fs/structure</tt>.</p>

      <p>Finally, read the well-documented API in
        <tt class="filename">subversion/include/svn_fs.h</tt>.</p>
    </div> <!-- server.fs.api (h4) -->

    <div class="h4" id="server.fs.struct" title="#server.fs.struct">
      <h4>Repository Structure</h4>
      

      <div class="h5" id="server.fs.struct.schema">
        <h5>Schema</h5>
        

        <p>
          To begin, please be sure that you're already casually familiar with
          Subversion's ideas of files, directories, and revision histories.  If
          not, see <a href="#model">Model &mdash; The versioning model used by Subversion</a>.  We can now offer precise,
          technical descriptions of the terms introduced there.</p>

        <!-- This is taken from jimb's very first Subversion spec! -->

        <pre>
A <strong class="firstterm">text string</strong> is a string of Unicode characters which is
canonically decomposed and ordered, according to the rules described in the
Unicode standard.

A <strong class="firstterm">string of bytes</strong> is what you'd expect.

A <strong class="firstterm">property list</strong> is an unordered list of properties.  A
<strong class="firstterm">property</strong> is a pair
<tt class="literal">(<em class="replaceable">name</em>,
  <em class="replaceable">value</em>)</tt>, where
<em class="replaceable">name</em> is a text string, and
<em class="replaceable">value</em> is a string of bytes.  No two properties in a
property list have the same name.

A <strong class="firstterm">file</strong> is a property list and a string of bytes.

A <strong class="firstterm">node</strong> is either a file or a directory.  (We define a
directory below.)  Nodes are distinguished unions &mdash; you can always tell
whether a node is a file or a directory.

A <strong class="firstterm">node table</strong> is an array mapping some set of positive
integers, called <strong class="firstterm">node numbers</strong>, onto
<strong class="firstterm">nodes</strong>.  If a node table maps some number
<em class="replaceable">i</em> to some node <em class="replaceable">n</em>, then
<em class="replaceable">i</em> is a <strong class="firstterm">valid node number</strong> in
that table, and <strong class="firstterm">node</strong> <em class="replaceable">i</em>is
<em class="replaceable">n</em>.  Otherwise, <em class="replaceable">i</em> is an
<strong class="firstterm">invalid node number</strong> in that table.

A <strong class="firstterm">directory entry</strong> is a triple
<tt class="literal">(<em class="replaceable">name</em>, <em class="replaceable">props</em>,
  <em class="replaceable">node</em>)</tt>, where
<em class="replaceable">name</em> is a text string,
<em class="replaceable">props</em> is a property list, and
<em class="replaceable">node</em> is a node number.

A <strong class="firstterm">directory</strong> is an unordered list of directory entries,
and a property list.

A <strong class="firstterm">revision</strong> is a node number and a property list.

A <strong class="firstterm">history</strong> is an array of revisions, indexed by a
contiguous range of non-negative integers containing 0.

A <strong class="firstterm">repository</strong> consists of node table and a history.

</pre>

        <!-- Some definitions: we say that a node @var{n} is a @dfn{direct
        child} of a directory @var{d} iff @var{d} contains a directory entry
        whose node number is @var{n}. A node @var{n} is a @dfn{child} of a
        directory @var{d} iff @var{n} is a direct child of @var{d}, or if there
        exists some directory @var{e} which is a direct child of @var{d}, and
        @var{n} is a child of @var{e}. Given this definition of ``direct
        child'' and ``child,'' the obvious definitions of ``direct parent'' and
        ``parent'' hold.

        In these restrictions, let @var{r} be any repository.  When we refer,
        implicitly or explicitly, to a node table without further
        clarification, we mean @var{r}'s node table.  Thus, if we refer to ``a
        valid node number'' without specifying the node table in which it is
        valid, we mean ``a valid node number in @var{r}'s node table''.
        Similarly for @var{r}'s history. -->

        <p>Now that we've explained the form of the data, we make some
          restrictions on that form.</p>

        <p><strong>Every revision has a root
            directory.</strong>  Every revision's node number is a valid node
          number, and the node it refers to is always a directory.  We call
          this the revision's <strong class="firstterm">root directory</strong>.</p>

        <p><strong>Revision 0 always contains an empty root
            directory.</strong>  This baseline makes it easy to check out
          whole projects from the repository.</p>

        <p><strong>Directories contain only valid
            links.</strong> Every directory entry's
          <em class="replaceable">node</em> is a valid node number.</p>

        <p><strong>Directory entries can be identified by
            name.</strong> For any directory <em class="replaceable">d</em>,
          every directory entry in <em class="replaceable">d</em> has a distinct
          name.</p>

        <p><strong>There are no cycles of
            directories.</strong>  No node is its own child.</p>

        <p><strong>Directories can have more than one
            parent.</strong>  The Unix file system does not allow more than
          one hard link to a directory, but Subversion does allow the analogous
          situation.  Thus, the directories in a Subversion repository form a
          directed acyclic graph (<strong class="firstterm">DAG</strong>), not a tree.
          However, it would be distracting and unhelpful to replace the
          familiar term &ldquo;directory tree&rdquo; with the unfamiliar term
          &ldquo;directory DAG&rdquo;, so we still call it a &ldquo;directory
          tree&rdquo; here.</p>

        <p><strong>There are no dead nodes.</strong>  Every
          node is a child of some revision's root directory.</p>

        <!-- </jimb> -->
      </div> <!-- server.fs.struct.schema (h5) -->

      <div class="h5" id="server.fs.struct.bubble-up">
        <h5>Bubble-Up Method</h5>
        

        <p>This section provides a conversational explanation of how the
          repository actually stores and revisions file trees.  It's not
          critical knowledge for a programmer using the Subversion Filesystem
          API, but most people probably still want to know what's going on
          &ldquo;under the hood&rdquo; of the repository.</p>

        <p>Suppose we have a new project, at revision 1, looking like this
          (using CVS syntax):</p>

        <pre>
prompt$ svn checkout myproj
U myproj/
U myproj/B
U myproj/A
U myproj/A/fish
U myproj/A/fish/tuna
prompt$
</pre>

        <p>Only the file <tt class="filename">tuna</tt> is a regular file,
          everything else in myproj is a directory.</p>

        <p>Let's see what this looks like as an abstract data structure in
          the repository, and how that structure works in various operations
          (such as update, commit, and branch).</p>

        <p>In the diagrams that follow, lines represent parent-to-child
          connections in a directory hierarchy.  Boxes are "nodes".  A node is
          either a file or a directory &ndash; a letter in the upper left
          indicates which kind.  A file node has a byte-string for its content,
          whereas directory nodes have a list of dir_entries, each pointing to
          another node.</p>

        <p>Parent-child links go both ways (i.e., a child knows who all its
          parents are), but a node's name is stored only in its parent, because
          a node with multiple parents may have different names in different
          parents.</p>

        <p>At the top of the repository is an array of revision numbers,
          stretching off to infinity.  Since the project is at revision 1, only
          index 1 points to anything; it points to the root node of revision 1
          of the project:</p>

        <pre>
                    ( myproj's revision array )
       ______________________________________________________
      |___1_______2________3________4________5_________6_____...
          |
          |
       ___|_____
      |D        |
      |         |
      |   A     |      /* Two dir_entries, `A' and `B'. */
      |    \    |
      |   B \   |
      |__/___\__|
        /     \
       |       \
       |        \
    ___|___   ___\____
   |D      | |D       |
   |       | |        |
   |       | | fish   |   /* One dir_entry, `fish'. */
   |_______| |___\____|
                  \
                   \
                 ___\____
                |D       |
                |        |
                | tuna   |  /* One dir_entry, `tuna'. */
                |___\____|
                     \
                      \
                    ___\____
                   |F       |
                   |        |
                   |        |   /* (Contents of tuna not shown.) */
                   |________|

</pre>

        <p>What happens when we modify <tt class="filename">tuna</tt> and commit?
          First, we make a new <tt class="filename">tuna</tt> node, containing the
          latest text.  The new node is not connected to anything yet, it's
          just hanging out there in space:</p>

        <pre>
                         ________
                        |F       |
                        |        |
                        |        |
                        |________|
</pre>

        <p>Next, we create a <em>new</em> revision of its parent
          directory:</p>

        <pre>
                 ________
                |D       |
                |        |
                | tuna   |
                |___\____|
                     \
                      \
                    ___\____
                   |F       |
                   |        |
                   |        |
                   |________|
</pre>

        <p>We continue up the line, creating a new revision of the next
          parent directory:</p>

        <pre>
              ________
             |D       |
             |        |
             | fish   |
             |___\____|
                  \
                   \
                 ___\____
                |D       |
                |        |
                | tuna   |
                |___\____|
                     \
                      \
                    ___\____
                   |F       |
                   |        |
                   |        |
                   |________|
</pre>

        <p>Now it gets more tricky: we need to create a new revision of the
          root directory.  This new root directory needs an entry to point to
          the &ldquo;new&rdquo; directory A, but directory B hasn't changed at
          all.  Therefore, our new root directory also has an entry that still
          points to the <em>old</em> directory B node!</p>

        <pre>
       ______________________________________________________
      |___1_______2________3________4________5_________6_____...
          |
          |
       ___|_____             ________
      |D        |           |D       |
      |         |           |        |
      |   A     |           |   A    |
      |    \    |           |    \   |
      |   B \   |           |   B \  |
      |__/___\__|           |__/___\_|
        /     \               /     \
       |    ___\_____________/       \
       |   /    \                     \
    ___|__/   ___\____              ___\____
   |D      | |D       |            |D       |
   |       | |        |            |        |
   |       | | fish   |            | fish   |
   |_______| |___\____|            |___\____|
                  \                     \
                   \                     \
                 ___\____              ___\____
                |D       |            |D       |
                |        |            |        |
                | tuna   |            | tuna   |
                |___\____|            |___\____|
                     \                     \
                      \                     \
                    ___\____              ___\____
                   |F       |            |F       |
                   |        |            |        |
                   |        |            |        |
                   |________|            |________|

</pre>

        <p>Finally, after all our new nodes are written, we finish the
          &ldquo;bubble up&rdquo; process by linking this new tree to the next
          available revision in the history array.  In this case, the new tree
          becomes revision 2 in the repository.</p>

        <pre>
       ______________________________________________________
      |___1_______2________3________4________5_________6_____...
          |        \
          |         \__________
       ___|_____             __\_____
      |D        |           |D       |
      |         |           |        |
      |   A     |           |   A    |
      |    \    |           |    \   |
      |   B \   |           |   B \  |
      |__/___\__|           |__/___\_|
        /     \               /     \
       |    ___\_____________/       \
       |   /    \                     \
    ___|__/   ___\____              ___\____
   |D      | |D       |            |D       |
   |       | |        |            |        |
   |       | | fish   |            | fish   |
   |_______| |___\____|            |___\____|
                  \                     \
                   \                     \
                 ___\____              ___\____
                |D       |            |D       |
                |        |            |        |
                | tuna   |            | tuna   |
                |___\____|            |___\____|
                     \                     \
                      \                     \
                    ___\____              ___\____
                   |F       |            |F       |
                   |        |            |        |
                   |        |            |        |
                   |________|            |________|

</pre>

        <p>Generalizing on this example, you can now see that each
          &ldquo;revision&rdquo; in the repository history represents a root
          node of a unique tree (and an atomic commit to the whole filesystem.)
          There are many trees in the repository, and many of them share
          nodes.</p>

        <p>Many nice behaviors come from this model:</p>

        <ol>
          <li><p><strong>Easy reads.</strong>  If a
              filesystem reader wants to locate revision
              <em class="replaceable">X</em> of file <tt class="filename">foo.c</tt>,
              it need only traverse the repository's history, locate revision
              <em class="replaceable">X</em>'s root node, then walk down the tree
              to <tt class="filename">foo.c</tt>.</p></li>

          <li><p><strong>Writers don't interfere with
                readers.</strong>  Writers can continue to create new nodes,
              bubbling their way up to the top, and concurrent readers cannot
              see the work in progress.  The new tree only becomes visible to
              readers after the writer makes its final &ldquo;link&rdquo; to
              the repository's history.</p></li>

          <li><p><strong>File structure is
                versioned.</strong>  Unlike CVS, the very structure of each
              tree is being saved from revision to revision.  File and
              directory renames, additions, and deletions are part of the
              repository's history.</p></li>
        </ol>

        <p>Let's demonstrate the last point by renaming the
          <tt class="filename">tuna</tt> to <tt class="filename">book</tt>.</p>

        <p>We start by creating a new parent &ldquo;fish&rdquo; directory,
          except that this parent directory has a different dir_entry, one
          which points the <em>same</em> old file node, but has a
          different name:</p>

        <pre>
       ______________________________________________________
      |___1_______2________3________4________5_________6_____...
          |        \
          |         \__________
       ___|_____             __\_____
      |D        |           |D       |
      |         |           |        |
      |   A     |           |   A    |
      |    \    |           |    \   |
      |   B \   |           |   B \  |
      |__/___\__|           |__/___\_|
        /     \               /     \
       |    ___\_____________/       \
       |   /    \                     \
    ___|__/   ___\____              ___\____
   |D      | |D       |            |D       |
   |       | |        |            |        |
   |       | | fish   |            | fish   |
   |_______| |___\____|            |___\____|
                  \                     \
                   \                     \
                 ___\____              ___\____      ________
                |D       |            |D       |    |D       |
                |        |            |        |    |        |
                | tuna   |            | tuna   |    | book   |
                |___\____|            |___\____|    |_/______|
                     \                     \         /
                      \                     \       /
                    ___\____              ___\____ /
                   |F       |            |F       |
                   |        |            |        |
                   |        |            |        |
                   |________|            |________|
</pre>

        <p>From here, we finish with the bubble-up process.  We make new
          parent directories up to the top, culminating in a new root directory
          with two dir_entries (one points to the old &ldquo;B&rdquo; directory
          node we've had all along, the other to the new revision of
          &ldquo;A&rdquo;), and finally link the new tree to the history as
          revision 3:</p>

        <pre>
       ______________________________________________________
      |___1_______2________3________4________5_________6_____...
          |        \        \_________________
          |         \__________               \
       ___|_____             __\_____        __\_____
      |D        |           |D       |      |D       |
      |         |           |        |      |        |
      |   A     |           |   A    |      |   A    |
      |    \    |           |    \   |      |    \   |
      |   B \   |           |   B \  |      |   B \  |
      |__/___\__|           |__/___\_|      |__/___\_|
        /  ___________________/_____\_________/     \
       |  / ___\_____________/       \               \
       | / /    \                     \               \
    ___|/_/   ___\____              ___\____      _____\__
   |D      | |D       |            |D       |    |D       |
   |       | |        |            |        |    |        |
   |       | | fish   |            | fish   |    | fish   |
   |_______| |___\____|            |___\____|    |___\____|
                  \                     \             \
                   \                     \             \
                 ___\____              ___\____      ___\____
                |D       |            |D       |    |D       |
                |        |            |        |    |        |
                | tuna   |            | tuna   |    | book   |
                |___\____|            |___\____|    |_/______|
                     \                     \         /
                      \                     \       /
                    ___\____              ___\____ /
                   |F       |            |F       |
                   |        |            |        |
                   |        |            |        |
                   |________|            |________|

</pre>

        <p>For our last example, we'll demonstrate the way
          &ldquo;tags&rdquo; and &ldquo;branches&rdquo; are implemented in the
          repository.</p>

        <p>In a nutshell, they're one and the same thing.  Because nodes are
          so easily shared, we simply create a <em>new</em>
          directory entry that points to an existing directory node.  It's an
          extremely cheap way of copying a tree; we call this new entry a
          <strong class="firstterm">clone</strong>, or more colloquially, a &ldquo;cheap
          copy&rdquo;.</p>

        <p>Let's go back to our original tree, assuming that we're at
          revision 6 to begin with:</p>

        <pre>
       ______________________________________________________
    ...___6_______7________8________9________10_________11_____...
          |
          |
       ___|_____
      |D        |
      |         |
      |   A     |
      |    \    |
      |   B \   |
      |__/___\__|
        /     \
       |       \
       |        \
    ___|___   ___\____
   |D      | |D       |
   |       | |        |
   |       | | fish   |
   |_______| |___\____|
                  \
                   \
                 ___\____
                |D       |
                |        |
                | tuna   |
                |___\____|
                     \
                      \
                    ___\____
                   |F       |
                   |        |
                   |        |
                   |________|

</pre>

        <p>Let's &ldquo;tag&rdquo; directory A.  To make the clone, we
          create a new dir_entry <strong>T</strong> in our
          root, pointing to A's node:</p>

        <pre>
       ______________________________________________________
      |___6_______7________8________9________10_________11_____...
          |        \
          |         \
       ___|_____   __\______
      |D        | |D        |
      |         | |         |
      |   A     | |    A    |
      |    \    | |    |    |
      |   B \   | |  B |  T |
      |__/___\__| |_/__|__|_|
        /     \    /   |  |
       |    ___\__/   /  /
       |   /    \    /  /
    ___|__/   ___\__/_ /
   |D      | |D       |
   |       | |        |
   |       | | fish   |
   |_______| |___\____|
                  \
                   \
                 ___\____
                |D       |
                |        |
                | tuna   |
                |___\____|
                     \
                      \
                    ___\____
                   |F       |
                   |        |
                   |        |
                   |________|

</pre>

        <p>Now we're all set.  In the future, the contents of directories A
          and B may change quite a lot.  However, assuming we never make any
          changes to directory T, it will <em>always</em> point to
          a particular pristine revision of directory A at some point in time.
          Thus, T is a tag.</p>

        <p>(In theory, we can use some kind of authorization system to
          prevent anyone from writing to directory T.  In practice, a well-laid
          out repository should encourage &ldquo;tag directories&rdquo; to live
          in one place, so that it's clear to all users that they're not meant
          to change.)</p>

        <p>However, if we <em>do</em> decide to allow commits in
          directory T, and now our repository tree increments to revision 8,
          then T becomes a branch.  Specifically, it's a branch of directory A
          which shares history with A up to a certain point, and then
          &ldquo;broke off&rdquo; from the main line at revision 8.</p>
      </div> <!-- server.fs.struct.bubble-up (h5) -->

      <div class="h5" id="server.fs.struct.diffy-storage">
        <h5>Diffy Storage</h5>
        
          
        <p>You may have been thinking, &ldquo;Gee, this bubble up method
          seems nice, but it sure wastes a lot of space.  Every commit to the
          repository creates an entire line of new directory
          nodes!&rdquo;</p>

        <p>Like many other revision control systems, Subversion stores
          changes as differences.  It doesn't make complete copies of nodes;
          instead, it stores the <em>latest</em> revision as a full
          text, and previous revisions as a succession of reverse diffs (the
          word "diff" is used loosely here &ndash; for files, it means vdeltas,
          for directories, it means a format that expresses changes to
          directories).</p>
      </div> <!-- server.fs.struct.diffy-storage (h5) -->
    </div> <!-- server.fs.struct (h4) -->

    <div class="h4" id="server.fs.implementation" title="#server.fs.implementation">
      <h4>Implementation</h4>
      

      <p>For the initial release of Subversion,</p>

      <ul>
        <li><p>The filesystem will be implemented as a library on
            Unix.</p></li>

        <li><p>The filesystem's data will probably be stored in a
            collection of .db files, using the Berkeley Database library.
            
    (In the future, of course, contributors are free
                modify the Subversion filesystem to operate with more powerful
                SQL database.)
   (For more information, see
            <a href="http://www.sleepycat.com">http://www.sleepycat.com</a>.)</p></li>
      </ul>
    </div> <!-- server.fs.implementation (h4) -->
  </div> <!-- server.fs (h3) -->

  <div class="h3" id="server.libsvn_repos" title="#server.libsvn_repos">
    <h3>Repository Library</h3>
    

    <!-- Jimb, Karl:  Maybe we should turn this into a discussion about how the
    filesystem will use non-historical properties for internal ACLs, and how
    people can add "external" ACL systems via historical properties...? -->

    <p>A Subversion <strong class="firstterm">repository</strong> is a directory that
      contains a number of components:</p>

    <ul>
      <li><p>a versioned filesystem (typically a collection of .db
          files)</p></li>
      <li><p>some hook scripts (for executing before or after
          commits)</p></li>
      <li><p>a locking area (used by Berkeley DB or other
          processes)</p></li>
      <li><p>a configuration area (for changing global
          behaviors)</p></li>
    </ul>

    <p>The Subversion filesystem is just that: a filesystem.  But it's also
      useful to provide an API that acts at the level of the repository.  The
      repository library (<tt class="filename">libsvn_repos</tt>) does this.</p>

    <p>In particular, it wraps a few <tt class="filename">libsvn_fs</tt>
      routines, such as those for beginning and ending commits, so that
      hook-scripts can run.  A pre-commit-hook script might check for a valid
      log message, and a post-commit-hook script might send an email to a
      mailing list.</p>

    <p>Additionally, the repository library provides convenience routines
      for examining and manipulating the filesystem.  For example, a routine to
      generate a tree-delta by comparing two revisions, routines for
      constructing new transactions, routines for querying log messages, and
      routines for exporting and importing filesystem data.</p>
  </div> <!-- server.libsvn_repos (h3) -->
</div> <!-- server (h2) -->

  <div class="h2" id="license" title="#license">
  <h2>License &mdash; Copyright</h2>
  

  
    <p>Copyright &copy; 2000-2008 Collab.Net.  All rights reserved.</p>

    <p>This software is licensed as described in the file
      <tt class="filename">COPYING</tt>, which you should have received as part of
      this distribution.  The terms are also available at
      <a href="http://subversion.tigris.org/license-1.html">http://subversion.tigris.org/license-1.html</a>.  If newer
      versions of this license are posted there, you may use a newer version
      instead, at your option.</p>
  
</div> <!-- license (h2) -->


</div>
</body>
</html>
